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Lithos 102 (2008) 279 – 294
www.elsevier.com/locate/lithos
Mantle source volumes and the origin of the mid-Tertiary ignimbrite
flare-up in the southern Rocky Mountains, western U.S.
G. Lang Farmer a,⁎, Treasure Bailley a , Linda T. Elkins-Tanton b
b
a
Department of Geological Sciences and CIRES, University of Colorado, Boulder, 80309, USA
Department of Earth, Atmospheric, and Planetary Sciences, MIT, 77 Massachusetts Ave., Cambridge, MA 02139, USA
Received 24 November 2006; accepted 6 August 2007
Available online 24 August 2007
Abstract
Voluminous intermediate to silicic composition volcanic rocks were generated throughout the southern Rocky Mountains,
western U.S., during the mid-Tertiary “ignimbrite flare-up”, principally at the San Juan and Mogollon-Datil volcanic fields. At both
volcanic centers, radiogenic isotope data have been interpreted as evidence that 50% or more of the volcanic rocks (by mass) were
derived from mantle-derived, mafic parental magmas, but no consensus exists as to whether melting was largely of lithospheric or
sub-lithospheric mantle. Recent xenolith studies, however, have revealed that thick (N100 km), fertile, and hydrated continental
lithosphere was present beneath at least portions of the southern Rocky Mountains during the mid-Tertiary. The presence of such
thick mantle lithosphere, combined with an apparent lack of syn-magmatic extension, leaves conductive heating of lithospheric
mantle as a plausible method of generating the mafic magmas that fueled the ignimbrite flare-up in this inland region. To further
assess this possibility, we estimated the minimum volume of mantle needed to generate the mafic magmas parental to the preserved
mid-Tertiary igneous rocks. Conservative estimates of the mantle source volumes that supplied the Mogollon-Datil and San Juan
volcanic fields are ∼2 M km3 and ∼7 M km3, respectively. These volumes could have comprised only lithospheric mantle if at
least the lower ∼ 20 km of the mantle lithosphere beneath the entire southern Rocky Mountains region underwent partial melting
during the mid-Tertiary and if the resulting mafic magmas were drawn laterally for distances of up to ∼ 300 km into each center.
Such widespread melting of lithospheric mantle requires that the lithospheric mantle have been uniformly fertile and primed for
melting in the mid-Tertiary, a possibility if the lithospheric mantle had experienced widespread hydration and refrigeration during
early Tertiary low angle subduction. Exposure of the mantle lithosphere to hot, upwelling sub-lithospheric mantle during midTertiary slab roll back could have then triggered the mantle melting. While a plausible source for mid-Tertiary basaltic magmas in
the southern Rocky Mountains, lithospheric mantle could not have been the sole source for mafic magmas generated to the south in
that portion of the ignimbrite flare-up now preserved in the Sierra Madre Occidental of northern Mexico. The large mantle source
volumes (N 45 M km3) required to fuel the voluminous silicic ignimbrites deposited in this region (N 400 K km3) are too large to
have been accommodated within the lithospheric mantle alone, implying that melting in sub-lithospheric mantle must have played a
significant role in generating this mid-Tertiary magmatic event.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Ignimbrite flare-up; Mantle source volumes; Southern Rocky Mountains
⁎ Corresponding author.
E-mail address: [email protected] (G.L. Farmer).
0024-4937/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.lithos.2007.08.014
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280
G.L. Farmer et al. / Lithos 102 (2008) 279–294
1. Introduction
The voluminous “ignimbrite flare-up” that affected
much of western North America during the mid-Tertiary
remains one of the world's more enigmatic major continental igneous events (Noble, 1972; Swanson et al., 1978;
Lipman, 1992). While it has long been suggested that this
magmatism was related in some fashion to roll back of
shallowly subducting oceanic lithosphere of the Farallon
Plate (Lipman et al., 1971; Coney and Reynolds, 1977;
Elston, 1984b), the relative roles of the continental
lithosphere, the sub-continental mantle, and slab-derived
components as magmatic sources, and the exact processes
responsible for triggering the magmatism itself, remain
unclear (Best and Christiansen, 1991; Humphreys, 1995).
These issues are especially problematic for the portion of
the ignimbrite flare-up that occurred in the southern
Rocky Mountain region (Fig. 1). Here igneous activity,
including the ∼60 K km3 of intermediate to silicic
volcanic rocks preserved at the San Juan volcanic field in
southern Colorado, took place within thick Paleoproterozoic continental lithosphere at a present-day distance of
some 1000 km inboard of the western edge of the North
American continent (Lipman et al., 1970; Elston et al.,
1973; Mutschler et al., 1987). How was this voluminous
magmatism generated so far inland and from what
sources?
In this study we further address the possibility that
lithospheric mantle was the major source of mafic magmas parental to the mid-Tertiary magmatism in the
southern Rocky Mountains. In our approach, we use
minimum estimates of the volume of mantle that must
have been involved in producing the estimated volumes of
mid-Tertiary igneous rocks present this region. Based on
these “mantle source volume” estimates, we demonstrate
that lithospheric mantle could have been the dominant
source of mafic parent magmas in the southern Rocky
Mountains, but only if melting of the base of the
lithospheric mantle occurred beneath essentially the entire
present-day southern Rocky Mountains, and if the mafic
magmas so generated were focused over lateral distances
of up to at least 300 km into one of only two major midTertiary volcanic centers active in this region. However,
further to the south in the Sierra Madre Occidental of
northern Mexico, the volumes of silicic magma generated
in the mid-Tertiary require a mantle source volume that far
exceeds any reasonable minimum estimate of the volume
of lithospheric mantle that could have melted during the
Fig. 1. A) Location of southern Rocky Mountains, western United States, B) study area, including surface outcrop of mid-Tertiary volcanic rocks
(crosses) in southern Rocky Mountains. Large volume centers are San Juan (SJVF) and Mogollon-Datil (MDVF) volcanic fields. Smaller centers
include the Never Summer (NSVF), Latir (LVF) and Organ Needle (ONVF) volcanic fields. Circles represent maximum radii for cylindrical “mantle
source volumes” supplying the MDVF and SJVF in the case where magmatism is assumed to have been triggered by conductive heating of
lithospheric mantle (see text).
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G.L. Farmer et al. / Lithos 102 (2008) 279–294
mid-Tertiary, and so implies that upwelling sub-lithospheric mantle must also have been a source of the midTertiary basaltic magmas produced in this region.
2. Mid-Tertiary magmatism in southern Rockies
Widespread intermediate to silicic composition volcanism affected much of western U.S. and northern Mexico
during the mid-Tertiary (Elston, 1984a; Best and
Christiansen, 1991; Lipman, 1992). As with other silicic
large igneous provinces, such as the Early Cretaceous
Whitsunday Volcanic Province of eastern Australia
(Bryan et al., 2000), this “ignimbrite flare-up” was both
voluminous and long-lived. Over 500 K km3 of silicic
ignimbrites erupted in western North America from
∼50 Ma to ∼20 Ma, and, if estimates of the amounts of
intermediate composition volcanic rocks and associated
plutonic rocks are also included, at least 5 M km3 of
igneous rock were produced during this time interval
(Johnson, 1991).
In the southern Rocky Mountains, mid-Tertiary
volcanism occurred discontinuously throughout much
of Colorado and New Mexico (Fig. 1; Elston et al., 1973;
Mutschler et al., 1987) but was concentrated at two major
centers: the San Juan volcanic field (SJVF) in southwestern Colorado, and the Mogollon-Datil volcanic field
(MDVF) located some 300 km further south in
southwestern New Mexico (Fig. 1). The SJVF was the
site of dominantly intermediate to silicic composition
magmatism over a 17 m.y. period from ∼ 35 Ma to
∼ 18 Ma (Colucci et al., 1991; Lipman, 2007). However,
if older silicic magmatism in the Sawatch Range in
central Colorado is also considered, then igneous activity
in southern Colorado spanned a longer time interval of
almost 20 m.y. (Lipman, 2007). Volcanism in the SJVF
commenced with the construction of large stratovolcanoes constructed largely of potassic, intermediate
composition lava and breccias, but with compositions
ranging from basalt to rhyolite (Fig. 2; Colucci et al.,
1991; Parker et al., 2005; Lipman, 2007). The volumes
of older, dominantly intermediate composition volcanic
rocks are difficult to constrain due to erosion and burial
by younger ignimbrites, but have been estimated at up to
40 K km3 (Lipman et al., 1970) (Table 1). The dominantly intermediate composition volcanism in the SJVF
was supplanted after several million years by the
eruption of large ignimbrites and the construction of
related calderas. The ignimbrites vary from phenocrystpoor (5–10 vol.% crystals) units, often compositionally
zoned from rhyolite to dacite, to more compositionally
uniform, crystal rich dacites (up to 45 vol.% crystals),
such as the 27.6 m.y. old, Fish Canyon tuff (Lipman,
281
2007). Cumulative volume estimates for these younger
ignimbrites range up to ∼ 20 K km3, about half that of the
older, intermediate composition volcanic rocks (Lipman
et al., 1970).
In addition to the volcanism, there is abundant
geologic and geophysical evidence at the SJVF for the
emplacement of voluminous, shallow level intrusive
igneous rocks during the mid-Tertiary (Lipman, 2007).
Based on the − 300 mgal anomaly beneath the SJVF (the
largest known in the conterminous U.S.), some workers
have suggested that there must be ∼300 K km3 of low
density crustal material present beneath the SJVF (Roy
et al., 2004). This amount corresponds to ∼ 5× the total
amount of extrusive igneous rocks in this region, a
reasonable value for intraplate volcanic centers (White
et al., 2006). The range of chemical compositions of
these intrusive igneous rocks is not known, but where
observed are more mafic than the contemporaneous
ignimbrites (i.e. granodioritic; Lipman, 2007).
A similar magmatic history has been proposed for the
MDVF, where voluminous, dominantly intermediate
composition, volcanism was supplanted through time by
large volume ignimbrite eruptions and caldera formation
(Elston, 1984a). Igneous activity was essentially contemporaneous with that in Colorado, with high-K,
predominately basaltic andesite to andesite volcanism
(Fig. 2) occurring from ∼ 40 Ma to ∼ 36 Ma, followed by
bimodal basaltic andesite and silicic activity, including
large volume ignimbrite deposition and attendant caldera
formation, taking place episodically from ∼ 36 to
∼ 24 Ma (McIntosh et al., 1992). As at the SJVF field,
eruptive volume estimates are complicated by uncertainties stemming from burial and erosion of the volcanic
Fig. 2. Wt.% K2O vs. wt.% SiO2 for mid-Tertiary volcanism in the
southern Rocky Mountain region. Data obtained from the North
American volcanic and intrusive rock database (NAVDAT). Boundary
between high and medium K rocks from (Gill, 1981).
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G.L. Farmer et al. / Lithos 102 (2008) 279–294
Table 1
Estimated volcanic and intrusive igneous rock volumes and calculated mantle source volumes for mid-Tertiary volcanic centers in southern Rocky
Mountains
Mid-Tertiary Volcanic Centers Age
(Ma) a
San Juan volcanic field
Andesites
Crystal-rich dacites
Crystal poor rhyolites
Intrusive rocks
Total SJVF
Mogollon-Datil volcanic field
Andesites
Silicic ignimbrites
Intrusive rocks
Total MDVF
Sierra Madre Occidental
Silicic ignimbrites
Intrusive rocks
Total
Preserved volume Basalt volume-“closed” Basalt volume “open” “Closed” system “Open” system
(K km3) b
system (K km3) c
system (K km3) d
MSV (M km3) e MSV (M km3) e
35–18
40–36
36–24
40
10
10
300
360
69
43
87
520
719
41
14
20
310
384
1.3
0.8
1.6
9.5
13
0.8
0.3
0.4
5.6
7.0
10
9
95
114
17
39
165
221
10
13
98
121
0.3
0.3
3.0
3.6
0.2
0.2
1.7
2.1
393
1965
2358
1703
3406
5109
546
2027
2600
38–21
31
62
93
9.5
35
45
a
volcanic rock age ranges from McIntosh et al. (1992), Ferrari et al. (2002), Parker et al. (2005) and Lipman (2007).
estimates of preserved volcanic rock volumes from Lipman et al. (1970), McIntosh et al. (1992) and Aguirre-Diaz and Labarthe-Hernández
(2003). Intrusive rock volume for San Juan volcanic field from Roy et al. (2004); others estimated assuming Mint/Mext = 5.
c
“closed” system basalts volumes calculated assuming various rock lithologies represent products of fractional crystallization from basalt parental
magma without interaction with continental crust. Fxtl used for andesites (and intrusive rocks), crystal-rich dacites (and ignimbrites from MDVF and
SMO), and crystal poor rhyolites were 0.5, 0.8 and 0.9, respectively.
d
“open” system basalt volumes calculated using Eq. (2)(see text) with ρandesite = ρignimbrite = 2.6 g/cm3, ρbasalt = 3.0 g/cm3. Fxtl used for andesites
(and intrusive rocks), crystal-rich dacites (and ignimbrites from MDVF and SMO), and crystal poor rhyolites were 0.3, 0.43 and 0.63, respectively
(Parker et al., 2005).
e
“closed” and “open” system mantle source volumes calculated from respective basalt volumes using Eq. (1), with ρmantle = 3.3 g/cm3 and
assuming 5% melting (Fmelt = 0.05), by mass.
b
rocks, but at least 10 K km3 of intermediate volcanic
rocks, and 9 K km3 of silicic ignimbrites are preserved
(Table 1) (McIntosh et al., 1992). Voluminous shallow
intrusive igneous rocks associated with the mid-Tertiary
volcanism at the MDVF are known or inferred from
gravity data (Keller et al., 1998) but are only well
exposed in the eastern portions of the volcanic field
where post-igneous activity, Basin and Range extension
has exposed epizonal alkali granite to monzonite (Seager
and McCurry, 1988).
3. Existing models for origin of mid-Tertiary
magmatism
Exactly why voluminous, mid-Tertiary igneous
activity occurred in western North America, particularly
in the southern Rocky Mountains, remains a major
question. Was magmatism simply the product of normal
arc processes that occurred far inland because of a low
angle of subduction of oceanic lithosphere (Lipman
et al., 1971; Coney and Reynolds, 1977)? Or was magmatism triggered by roll back of the shallowly subduct-
ing oceanic lithosphere of the Farallon Plate (Coney,
1978; Lipman, 1980), or by local intraplate deformation
of deep continental lithosphere (Mutschler et al., 1987)?
Did lithospheric extension play a dominant role in
triggering magmatism, either by provoking decompression melting of sub-lithospheric mantle (McKenzie and
Bickle, 1988), or by inducing melting of mafic portions
of the continental lithospheric mantle (Leeman and
Harry, 1993)?
We accept here the premise that both the early,
dominantly intermediate volcanism and the subsequent
silicic magmatism at both the SJVF and MDVF were
ultimately related to injection of basaltic magmas into the
pre-existing continental crust (Johnson, 1991; Perry
et al., 1993). Evidence for such is largely based on the
fact that mid-Tertiary igneous rocks contain a large
proportion of basaltic material derived from partial
melting of the mantle at or near the time of magmatism
(Johnson, 1993). For example, Nd isotopic data suggest
that 50% or more of the mass of silicic ignimbrites at the
SJVF and MDVF originated from mantle-derived
basaltic magmas, with the remaining mass representing
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G.L. Farmer et al. / Lithos 102 (2008) 279–294
assimilated, pre-existing continental crust (Perry et al.,
1993). We also agree with previous workers that the
observed changes in the composition and style of volcanism through time at both locations were most likely the
result of changes in crustal composition and thermal
structure resulting from a protracted period of mafic magma injection, possibly combined with changes through
time in the flux of basaltic magma into the crust (Huppert
and Sparks, 1988; Lipman, 2007).
An important remaining issue, however, is determining where and how the basaltic magmas were generated,
in particular defining the relative roles of sub-lithospheric and lithospheric mantle in producing the mafic
magmas parental to the ignimbrite flare-up. In the Great
Basin and Sierra Madre Occidental most workers have
suggested that the mid-Tertiary magmatism was fueled
by mafic magmas derived from partial melting of the
sub-lithospheric mantle, the latter potentially contaminated by slab-derived components (Best and Christiansen, 1991; Ferrari et al., 2002). In the southern Rocky
Mountains, in contrast, the relatively low ɛNd values
(compared to mid-ocean ridge basalts) determined for
mid-Tertiary mafic igneous rocks (∼ 0; Perry et al.,
1993; Lawton and McMillan, 1999) have been interpreted as evidence of their derivation from low ɛNd
lithospheric mantle, albeit modified compositionally by
a pre-mid-Tertiary subduction event (Davis and Hawkesworth, 1993).
In this paper we further address the plausibility of
mantle lithosphere as a source for mid-Tertiary mafic
magmas in the southern Rocky Mountains. We were
prompted to reassess this issue because of recent studies
that revealed that thick lithospheric mantle was likely to
have been present beneath at least portions of this region in
the mid-Tertiary. For example, petrologic studies of
lithosphere-derived xenoliths entrained in mid-Tertiary
ultramafic diatremes on the Colorado Plateau, just west of
the SJVF, indicate a lithospheric thickness of at least
100 km at this time (Smith and Griffin, 2005). Current
estimates of present-day lithospheric thicknesses in the
southern Rocky Mountain area, based on passive seismic
data, are also at least 100 km (Yuan and Dueker, 2005).
Because significant decompression melting of dry peridotite at “normal” potential temperatures (∼1290 °C)
does not occur for lithospheric thicknesses greater than
∼80 km (McKenzie and Bickle, 1988), the existence of
thick lithosphere in the southern Rocky Mountains implies
that the conditions necessary for decompression melting
of sub-lithospheric mantle may not have been achieved
here during the mid-Tertiary. The fact that significant
lithospheric extension post-dated at least the initiation of
significant mid-Tertiary magmatism in the MDVF
283
(Cather, 1990; Chapin et al., 2004) and post-dated all of
the intermediate to silicic volcanism in the SJVF (Lipman
et al., 1970) also supports the idea that thick lithospheric
mantle could have been present beneath the southern
Rocky Mountains during the mid-Tertiary magmatism.
The lack of a clear relationship between lithospheric
extension and magmatism in the southern Rocky Mountains, particularly the absence of extension during the
SJVF magmatism, suggests that models for producing
intraplate magmatism through lithospheric extension
and melting of upwelling sub-lithospheric mantle may
not be relevant for the mid-Tertiary magmatism in this
region. Instead, models linking the mid-Tertiary magmatism to partial melting of continental lithospheric mantle during roll back of shallowing subducting oceanic
lithosphere become attractive (Lipman and Glazner,
1991; Lawton and McMillan, 1999), particularly if the
lithospheric mantle was fertile for basalt generation and
had been hydrated and refrigerated during Late Cretaceous/Early Tertiary low angle subduction (Dumitru
et al., 1991). Recent studies of Colorado Plateau
xenoliths, in fact, have now provided direct evidence
that refrigeration and hydration of the lithospheric
mantle did take place in the vicinity of the present-day
southern Rocky Mountains, that this hydration was Late
Cretaceous to Early Tertiary in age but affected Proterozoic age mantle lithosphere, and that the peridotitic
portions of the lithosphere were fertile with respect to
basalt generation in the mid-Tertiary (Lee et al., 2001;
Smith et al., 2004; Lee, 2005; Smith and Griffin, 2005).
These observations are all consistent with the possibility
that hydrated, peridotitic lithospheric mantle spawned
the magmas parental to the mid-Tertiary magmatism
(Lawton and McMillan, 1999; Smith and Griffin, 2005),
and with recent numerical models demonstrating that
dehydration of shallowly subducting oceanic lithosphere
during the Laramide orogeny could have occurred even
at distances as far inland as the present-day southern
Rocky Mountains (English et al., 2003).
4. Calculating mantle “source volumes”
To further assess the plausibility of a lithospheric
mantle source for magmas parental to the mid-Tertiary
southern Rocky Mountains we estimated the volume of
mantle that must have undergone partial melting in order
to fuel the mid-Tertiary volcanism (the mantle “source
volume”). The simple logic here is that if the mantle
source volume required exceeds estimates of the volume
of “lithospheric” mantle that could have been brought
under conditions of partial melting during mid-Tertiary
igneous activity in this region, then sub-lithospheric
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284
G.L. Farmer et al. / Lithos 102 (2008) 279–294
mantle must have also contributed to the magmatism.
Note that for our purposes, “lithospheric” mantle is considered to be the shallow, conducting portion of the
mantle remaining after removal of the Farallon Plate
from beneath the southern Rockies.
To calculate mantle source volumes, we first estimated the mass of basaltic magma required to produce the
amount of preserved volcanic rocks and unerupted
intrusive igneous rock (see below). We then calculated a
mantle source volume required to produce this igneous
rock using Eq. (1);
Vmantle ¼
Vbasalt qbasalt
⁎
Fmelt qmantle
ð1Þ
where Fmelt is the average mass fraction (or extent)
of mantle melting and ρmantle and ρbasalt are mantle
(∼ 3.3 g/cm 3 ) and basalt (∼ 3 g/cm 3 ) densities,
respectively.
This method of estimating mantle source volumes is
analogous to that used for large igneous provinces
(Coffin and Eldholm, 1993), but with the added
complications that basalt volumes must be calculated
from the preserved volumes of a diverse range of more
silicic composition extrusive and intrusive igneous
rocks which evolved in systems open to interaction with
pre-existing continental crust. As a result there is little
hope of obtaining an accurate determination of the
mantle source volume of the mid-Tertiary magmatism.
We cannot unambiguously determine the masses
and compositions of eroded or buried igneous rocks,
the amount of pyroclastic material dispersed from the
vent areas, or the open system differentiation histories
of all magma compositions produced as a function
of time and position at any given volcanic center.
We can, however, assess the implications even crude
minimum estimates of mantle source volume may
have for understanding the origin of the mid-Tertiary
magmatism.
For these minimum mantle source volume estimates
we restricted our attention to the preserved volcanic
rock volumes estimated for the SJVF and MDVF
(Table 1). Other mid-Tertiary volcanic centers in the
southern Rocky Mountains produced considerably
smaller volumes of erupted material. For example, the
Latir volcanic field in northern New Mexico only
produced an estimated 1000 km3 of silicic volcanic
rock (Lipman et al., 1986), while our estimate of
the total preserved erupted volcanic rock volumes at
the ∼ 28 Ma Never Summer volcanic field in northern
Colorado is only ∼ 15 km 3 . Overall, volcanism
peripheral to the SJVF and MDVF likely represents
only 10% of the volume of extrusive igneous rocks
in the southern Rocky Mountain area (Perry et al.,
1993) and for this reason was excluded from our mantle
source volume estimates.
Our minimum mantle source volume estimates for
the MDVF and SJVF are given in Table 1. For the
calculations shown, we assumed an average andesitic
composition for the early intermediate composition
volcanic rocks, largely because detailed estimates of the
relative amounts of volcanic rock as a function of bulk
composition are lacking at both volcanic centers. We
used a dacitic composition for ignimbrites at the MDVF
and assumed equal volumes of crystal rich dacites and
crystal poor rhyolites amongst the younger ignimbrites
at the SJVF (Lipman, 2007). We used a ratio between
intrusive and extrusive igneous rocks (Mint/Mext) equal
to five for both volcanic centers, as directly estimated
for the SJVF, and assumed that none of the intrusive
rocks represent cumulates related to the production of
the extrusive igneous rocks produced at either volcanic
field. In addition, we assigned an “andesitic” average
bulk composition to the intrusive igneous rocks
(∼60 wt.% SiO2), although direct constraints regarding
the actual average composition of these rocks do not
exist.
Based on the above assumptions, we show two example mantle source volume calculations in Table 1. In the
“closed system” estimate, each of the lithologies at each
volcanic center are considered to represent the products
of fractional crystallization with no crustal assimilation
(or equivalently, for our purposes, partial melting of
basaltic composition crustal rocks; Sisson et al., 2005).
In the example shown, the early andesites (and intrusive
igneous rocks), later dacitic ignimbrites, and later
rhyolitic ignimbrites are considered to be the products
of 50%, 80%, and 90% (respectively) fractional
crystallization from a basaltic parental magma (Bowen,
1928; Annen et al., 2006). In this case, the ratio of the
volume of basalt required to the volume of igneous rock
(Vbasalt/Vrock) increases from ∼2 for andesites to ∼9 for
rhyolites, corresponding to a total of ∼ 0.7 M km3 and
∼ 0.2 M km3 of parental basalts at the SJVF and MDVF,
respectively (assuming densities of 2.6 g/cm 3 for
andesites and dacites, and 3.0 g/cm3 for basalt). From
Eq. (1), calculated mantle source volumes for the SJVF
and MDVF are then ∼ 13 M km3 and ∼ 4 M km3. For this
calculation we assumed a mantle density of 3.3 g/cm3
and 5% mantle melting (Fmelt = 0.05). We consider the
latter value to be reasonable given the generally potassic
nature of the mid-Tertiary magmatism in the southern
Rockies (Fig. 2), which likely requires Fmelt to be 0.05 or
less (Hirose and Kushiro, 1993).
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G.L. Farmer et al. / Lithos 102 (2008) 279–294
While “closed system” fractionation represents an
obvious end-member for mantle source volume calculations, it is clearly not relevant for most of the midTertiary magmatism in the southern Rocky Mountains
given the abundant evidence that both intermediate and
silicic igneous rocks evolved in systems open to crustal
interaction (Johnson et al., 1990; Colucci et al., 1991;
Parker et al., 2005). As a result, the actual amount of
basaltic magma required to generate a given amount of
more silicic magma is likely to be less than that required
in the case of closed system fractional crystallization
(Bowen, 1928). The “open system” mantle source
volume calculation in Table 1 represents an effort to
account for wall–rock interaction during production of
magmas parental to the igneous rocks at both the SJVF
and MDVF.
Many assimilation/fractional crystallization models
have been proposed for production of continental
magmas (DePaolo, 1981; Bohrson and Spera, 2001).
But for the early, dominantly intermediate, composition igneous rocks we based our estimates of the
parental mafic magma volumes on direct observations
of the process as preserved at the Organ Needle
volcanic field located at the eastern periphery of the
MDVF (Fig. 1). Here field and geochemical observations suggest that basaltic magmas were present in the
mid-crust during the formation of more silicic magma,
and that these mafic magmas incorporated partial melts
of Precambrian granite country rocks (Verplanck et al.,
1999). The wall-rock assimilation produced a significant shift in the isotopic compositions of mafic magma
(ɛNd from − 2 to − 6), but only a modest shift in the bulk
composition of magma from basalt to basaltic andesite
(Verplanck et al., 1999). The basaltic andesite then
underwent fractional crystallization, with little additional crustal interaction, to produce a spectrum of
magmatic compositions, including 500 to 1000 km 3 of
∼ 70–77 wt.% SiO 2 ash flow tuff (Seager and
McCurry, 1988). Such a sequence of open system
magmatic differentiation is similar to that predicted by
Reiners et al. (1995), who suggested that mafic
magmas injected into the continental crust will likely
undergo an initial period of high assimilation, and
suppressed crystallization, rates, followed by higher
crystallization, and lower assimilation, rates once the
magma becomes saturated with plagioclase and/or
pyroxene. While we cannot necessarily extend this
model to all intermediate composition igneous rocks in
the southern Rocky Mountains, a two stage model in
which basaltic mafic magmas first incorporate crustal
melts and then undergo crystal fractionation to produce
a spectrum of melt compositions is a viable possibility
285
for the generation of the early volcanism, and one that
can be simply approximated by Eq. (2),
Vbasalt ¼ Vrock ⁎
qrock
1
⁎
qbasalt ð1 þ Fcrust Þ⁎ð1 Fxtl Þ
ð2Þ
In this expression, Vbasalt is the volume of parental
basaltic rock, Vrock is the final igneous rock volume of a
given bulk composition, Fcrust is the mass fraction of
assimilated pre-existing crust in that volcanic rock
(measured relative to original basalt magma mass; i.e.
grams crust/grams basalt), Fxtl is the mass of fractionated crystalline material from the crustally contaminated
magma, and ρrock, ρbasalt are densities of more silicic
rock (e.g. andesite, dacite, rhyolite) and basaltic rock,
respectively.
To estimate the basalt volumes needed to supply the
early andesites (and “andesitic” composition intrusive
rocks) at both the MDVF and SJVF, we used direct
observations from the Organ Needle volcanic field that
basaltic andesite magmas here were generated by
incorporation of ∼20% (Fcrust = 0.2) partial melt of
Precambrian granite wall rocks (Verplanck et al., 1999).
We then used estimates made for early andesites from the
eastern SJVF (Conejos volcanic rocks) that require ∼30%
fractionation of crystalline material from parental basaltic
andesites (∼52 wt.% SiO2) to produce andesite (60 wt.%
SiO2) (Parker et al., 2005). This calculation produces a
Vbasalt/Vrock of ∼1, about half that required for production
of andesitic magmas from closed system fractional
crystallization of a basaltic parental magma.
For the younger, large volume ignimbrites, we distinguished between crystal poor rhyolites and the crystalrich dacites at the SJVF where information regarding the
volumes of these different ignimbrite types is available
(Lipman, 2007). The crystal rich dacites likely represent
re-mobilization of previously generated igneous material
induced by mafic magma injection into the upper crust
(Bachmann et al., 2002). As a result, these ignimbrites
contain little new addition of new basaltic material,
although basalt representing some 40% of the heated
mush volume may be required to supply sufficient heat to
re-mobilize (Bachmann and Bergantz, 2003). However,
while the crystal-rich ignimbrites may not be direct
products of open system differentiation of basaltic magmas, the erupted crystal mushes have isotopic compositions that overlap those of older intermediate composition
volcanic rocks (Perry et al., 1993) which suggest that
these mushes were, like the older volcanic rocks, derived
from parental basaltic magmas that underwent significant
crustal interaction. Isotopic data from ignimbrites
throughout the Rockies have led to estimates of crustal
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components in these rocks from 20 wt.% to 50 wt.%
(Perry et al., 1993), which, although highly dependent on
the isotopic compositions of crustal assimilant and the
original basaltic magma, are also similar to the amounts of
crust estimated to be present in the older andesitic rocks.
So for our purposes we consider the crystal-rich dacites to
represent re-mobilization of older, either erupted or
unerupted igneous materials (melts and/or cumulates),
that were originally produced by open system evolution of
basaltic magmas in a manner similar to that outlined for
the older andesitic rocks. Assuming an average dacitic
composition for the crystal-rich ignimbrites (Fxtl = 0.43;
Parker et al., 2005), and 20% crustal assimilation, leads to
a Vbasalt/Vrock∼1.5. If the mass of basaltic magma needed
to re-mobilize the crystal mush that actually erupted is
included in this estimate, then the total Vbasalt/Vrock
represented by the younger ignimbrites could reach as
high as ∼2.
In contrast, we assumed that the crystal poor rhyolites
were the direct products of protracted crystal fractionation from a crustally contaminated basalt andesite
parental magma. Crystal fractionation models for the
Conejos volcanic rocks (Parker et al., 2005) suggest that
∼ 63% fractionation is required to drive basaltic andesite
to a rhyolitic (∼70 wt.% SiO2) composition. Assuming
20% (by mass) crustal assimilation requires a Vbasalt/
Vrock ∼ 2 for the crystal poor rhyolites.
Using the above considerations, our “open system”
estimate of the total volumes of basaltic magma required
to supply the preserved volcanic rocks at the SJVF and
MDVF are ∼0.4 M km3 and ∼0.1 M km3, respectively
(Table 1). For this calculation, we assumed a dacitic
composition for the younger ignimbrites at the MDVF,
given that relative volume estimates for crystal rich dacites
and crystal poor rhyolites are not available at this volcanic
center. With 5% mantle melting, these basalt volumes
translate into mantle source volumes for the SJVF and
MDVF of 7 M km3 and 2 M km3, respectively (Table 1).
The basalt volumes and mantle source volumes
estimated in this fashion are significantly lower than
those determined for the closed system magmatic
differentiation case. Nevertheless, it is worth noting our
“open system” estimate of the volume of basaltic magma
required just for the SJVF is similar to estimates of total
volumes of basalt produced at small large igneous
provinces (LIPS), such as the Columbia River basalts
(Courtillot and Renne, 2003), although the latter produced
∼0.2 M km3 of basaltic magma over an interval of only
∼1 m.y., as opposed to the 15–20 m.y. duration of
magmatism at the SJVF.
While we consider the “open system” estimates given in
Table 1 as our best estimates of the minimum mantle
Fig. 3. Mantle source volume (M km3) vs. eruptive volume (K km3) for
San Juan volcanic field. Relative proportions of andesite, dacites, and
rhyolites and Fxtl for each composition as in Table 1, except where
otherwise noted. Intrusive rocks assumed to be solidified andesitic
magmas. The “MSV” (mantle source volume) limit shown in each
panel represents the maximum volume of lithospheric mantle that could
have provided melt to the San Juan volcanic field based on the spacing
between, and duration of igneous activity at, the major volcanic centers
in the southern Rocky Mountains (see text). A) effect of varying mass
of assimilated crust (Fcrust) on calculated mantle source volume for
Fcrust = 0 (“closed” system case, Table 1) and Fcrust = 0.2 (“open” system
case, Table 1). Also shown are effects on “open” system mantle source
volume calculated in Table 1 of uniformly changing Fxtl by 20%
for each magma composition, B) effect of varying mass fraction
of mantle melting, Fmelt, on open system mantle source volume. The
latter calculated for Fcrust = 0.2 and Mint/Mext = 5, C) effect of varying
Mint/Mext on “open” system mantle source volume (Fcrust = 0.2,
Fmelt = 0.05).
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G.L. Farmer et al. / Lithos 102 (2008) 279–294
source volume required to fuel the mid-Tertiary magmatism in the southern Rocky Mountain, these estimates are
obviously sensitive not only to the total eruptive volume
and the fraction of assimilated crust used for a given
volcanic center (Fig. 3A), but also the degree of mantle
melting (Fig. 3B) and the relative proportions of intrusive
vs. extrusive igneous rocks (Mint/Mext; Fig. 3C). For
example, reducing Mint/Mext by a factor of 2.5 from 5 to 2
(c.f. Lipman, 2007) at the SJVF reduces the calculated
mantle source volume here from 7 M km3 to 3.6 km3
(Fig. 3C). In contrast, even 20% variations in Fxtl used for
various extrusive rock lithologies at the SJVF produce only
modest variations in the calculated mantle source volume
(Fig. 3A). Nevertheless, given that the volume of eruptive
rock, Mint/Mext, and the average fraction of mantle melting
could easily differ by a factor of two relative from the
values used in Table 1 for either the MDVF or SJVF, the
minimum mantle source volume for these volcanic fields
could differ by at least a factor of two from the values given
in Table 1. But it should be emphasized that our open
system approach towards generating parental basalt
volumes yields considerably lower Vbasalt/Vrock than
conventional assimilation–fractional crystallization models. When applied to the evolution of Tertiary intermediate
to silicic volcanic rocks at the Latir volcanic field, for
example, assimilation–fractional crystallization models
yield Vbasalt/Vrock ∼4–5 (Johnson et al., 1990), as opposed
to the values of 1–2 generated here (Table 1). Therefore, we
consider our minimum estimates of parental basalt
volumes, and of mantle source volumes, to represent
conservative lower limits for both parameters.
287
5. Results and discussion
5.1. A lithospheric mantle source for the ignimbrite
flare-up?
Armed with minimum mantle source volume estimates
for the MDVF and SJVF, we now address the potential
role of conductive heating of the mantle lithosphere in
producing the mid-Tertiary magmatism (cf. Turner et al.,
1996). For this purpose we assume for simplicity that each
volcanic center in the southern Rocky Mountains tapped a
cylindrical mantle source volume within the continental
lithospheric mantle (CLM) centered directly beneath each
center with a height and radius sufficient to attain the
required mantle source volume (Figs. 1 and 4), essentially
implying a homogeneous and isotropic mantle in terms of
magma transport. The heights of these cylinders are
limited by the minimum depth in the CLM that could be
heated above the mantle solidus temperature during the
15–20 m.y. time interval over which igneous activity at
the SJVF and MDVF was active. Over 20 m.y., a thermal
pulse initiated at the base of the lithosphere could migrate
upwards ∼20 km, assuming a uniform thermal diffusivity
in the mantle of 10− 6 m2/s (Fig. 5). Assuming that the
increase in temperature imparted by this thermal pulse
was sufficient to initiate melting (see following discussion), ∼20 km represents an estimate of the maximum
thickness of lithospheric mantle that could have partially
melted beneath the southern Rocky Mountains during the
mid-Tertiary purely as a result of conductive heating (i.e.
ignoring heat advected by magmas rising through the
Fig. 4. A) Height (km) vs. radius (km) of a cylindrical mantle “source volume” (B) centered beneath a given volcanic center and rooted at base of preexisting mantle lithosphere. Solid lines are isovolumetric curves for mantle source volumes ranging from 1 to 100 M km3. Also shown are
isovolumetric curves for mantle source volumes calculated for the Mogollon-Datil volcanic field (MDVF), San Juan volcanic field (SJVF), and Sierra
Madre Occidental (SMO) in the “open system” case shown in Table 1. The zone of potential melting of hydrous mantle during conductive heating of
the continental lithospheric mantle (CLM) is based on the depth of intersection between steady-state conductive geotherm and water-saturated
peridotite solidus shown in Fig. 5B.
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G.L. Farmer et al. / Lithos 102 (2008) 279–294
Fig. 5. Diagrammatic representation of thermal structure of lithosphere beneath present-day southern Rocky Mountains during the Late Cretaceous/
Early Tertiary “Laramide” orogeny (A) and mid-Tertiary ignimbrite flare-up (B). Dashed lines on both figures are steady-state geotherms (ignoring
internal heat production in lithosphere) for the case where temperatures at base of lithosphere (either base of Farallon lithosphere (A) or base of
continental lithospheric mantle (CLM; (B)) are imposed by “normal” potential temperature (1290 °C) convecting mantle. The “20 m.y. geotherm”
shown in (B) was estimated using constant temperature boundary condition for instantaneous heating at base of lithosphere (Carslaw and Jaeger,
1959), using a uniform mantle thermal diffusivity of 10− 6 m2/s. Dry and various wet solidi from Mysen and Boettcher (1975), Hirschmann et al.
(1999), Hirschmann (2000), and Ohtani et al. (2004). Cartoons at right of diagram show possible disposition of continental crust, sub-continental
lithospheric mantle (CLM), subducted oceanic lithosphere (Farallon lithosphere) and underlying convecting mantle during Laramide orogeny (C) and
mid-Tertiary (D). CLM in both cartoons is transparent, except for metasomatic veins related to dehydration (C) and for cylinders representing “mantle
source volumes” for basaltic magmas underlying the southern Rocky Mountain region (D) related to conductive heating and melting of CLM during
mid-Tertiary “roll back” of the Farallon lithosphere.
lithosphere). If lithospheric mantle melting at both the
SJVF and MDVF were restricted to columns with radii
similar to that prescribed at the surface by the volcanic
centers themselves (∼100 km, prior to disruption by
Basin and Range extension; Fig. 1), then a maximum of
∼1 M km3 of lithospheric mantle could have melted
beneath each (Fig. 4). This volume of mantle is a factor of
∼2 and ∼7 lower than the “open system” mantle source
volumes we estimated for the MDVF and SJVF,
respectively (Table 1), implying that sufficient basaltic
magma could only have been supplied to each volcanic
center if each drew magma from a wider area than defined
by the present-day surface “footprints” of the volcanic
fields.
Just how wide a lateral reach for magma is required?
The maximum radius of this cylinder is difficult to assess,
as it is ultimately controlled by how far magma can be
transported laterally through the lithospheric mantle.
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G.L. Farmer et al. / Lithos 102 (2008) 279–294
Few studies have addressed this issue directly. Certainly
evidence exists for long-distance transport of basaltic
magma through the mid to upper continental crust, with
transport distance of up to 2000 km having been suggested
for some terrestrial giant radiating dike swarms (Ernst
et al., 1995). But the length scales for lateral melt migration
via porous flow in the upper mantle are not well-defined,
although in plume-ridge systems lateral transport distances
for melt of 100 to 1000 km have been proposed (Schilling,
1985; Braun and Sohn, 2003). For our purposes, however,
we can at least set a maximum possible “reach” for the
SJVF and MDVF based on the fact that the source regions
of adjacent major volcanic centers cannot overlap, by
analogy with groundwater flowing through multiple drains
(Bear, 1972). By this criterion, the maximum lateral
reaches of the SJVF and MDVF are attained when their
mantle source volume are mutually tangent (Fig. 1).
Because the mantle source volume of the SJVF is ∼3.5
times larger than that of the MDVF (Table 1), this situation
corresponds to cylinders with radii of ∼350 km and
∼190 km, respectively (Fig. 1). The maximum reach for
each volcanic center therefore limits its maximum source
volume. Using a 20 km thick mantle source volume for
both the SJVF and MDVF yields maximum mantle source
volumes of ∼7.7 M km3 and ∼2.3 M km3, respectively.
The maximum mantle source volumes for the SJVF
and MDVF based on the above physical constraints are
similar to the “open system” minimum mantle source
volume values we estimated independently on the basis of
igneous rock volumes (Figs. 3 and 4; Table 1). The implication is, then, that in order for the mid-Tertiary
magmatism to have originated via conductive heating of
pre-existing mantle lithosphere, the lowermost 20 km of
the lithosphere must have undergone an average of at least
5% partial melting beneath essentially the entire
∼300,000 km2 present-day area of the southern Rockies
in Colorado and New Mexico. The resulting magmas
must have then migrated laterally up to 300 km or more in
the deep lithosphere while being focused on one of only
two volcanic centers, either the SJVF or the MDVF. This
model also requires that the lower 20 km of the lithospheric mantle have everywhere been fertile for basalt
generation, and that the thermal pulse responsible for
triggering conductive melting, whatever its origin, must
have been sufficient to generate partial melting at base of
the mantle throughout the southern Rocky Mountain area.
Removal of shallowly subducting oceanic lithosphere
from the base of the CLM, and exposure of the mantle
lithosphere to upwelling sub-lithospheric mantle is one
method of actually satisfying the above conditions. As
mentioned earlier, the emplacement of cold oceanic
lithosphere beneath the Rocky Mountain area not only
289
results in conductive cooling of the continental lithosphere but also in significant aqueous metasomatism of
the base of the pre-existing continental mantle lithosphere, even at distances as far inland as the present-day
southern Rocky Mountains (Dumitru et al., 1991;
English et al., 2003). Critical for our purposes is the
fact that volatile addition considerably lowers the solidus
of the mantle rocks (Hirschmann, 2006) and essentially
primes the pre-existing mantle lithosphere for melting
(Lipman and Glazner, 1991). To illustrate this effect,
consider the case where ∼100 km thick oceanic lithosphere is inserted during low angle subduction between
pre-existing mantle lithosphere (∼ 150 km thick) and
underlying convecting mantle with normal potential temperatures (1290 °C). By the time a steady-state conductive geotherm has been reestablished in the subcontinental mantle and underlying oceanic lithosphere,
the pre-existing continental mantle lithosphere has
cooled to temperatures well below even the watersaturated peridotite solidus (point 1, Fig. 5A, B). The
result is that addition of volatiles to the base of the
continental lithosphere does not immediately trigger
mantle melting. Instead, melting of the lithosphere is
delayed until the oceanic lithosphere is removed, for
example, via delamination (Elkins-Tanton, 2005). Removal of the underlying oceanic lithosphere exposes the
cold but hydrated mantle lithosphere to upwelling sublithospheric mantle, ultimately reestablishing the prelow angle subduction temperature gradient within the
lithospheric mantle (point 2, Fig. 5B). Depending on the
extent of hydration, the temperature of the lower reaches
of the lithosphere can exceed the mantle solidus temperature and melting ensues. Such hydrous melting is likely
to be of low productivity (b5%)(Hirschmann et al.,
1999) but the total volume of melt produced depends on
the volume of mantle that experiences supersolidus
temperatures as a steady-state geotherm is reestablished
in the mantle lithosphere. In general, for unsaturated
conditions, the more water added, the more the peridotite
solidus shifts to lower temperatures (Hirschmann, 2006;
Liu et al., 2006) and the greater the volume of mantle that
could potentially be involved in hydrous melting. For
example, 20 m.y. after the heating of the base of the
CLM, at least the lower ∼20 km of lithospheric mantle
containing 2% H2O could achieve temperatures exceeding the relevant hydrous peridotite solidus, even without
calling upon upwelling of anomalously high potential
temperature mantle (Fig. 5B). In addition, because the
base of the mantle lithosphere heats relatively rapidly,
compared to shallower mantle depths (Turner et al.,
1996), temperatures at the base of the CLM approximate
their steady-state values after 20 m.y. has elapsed and so
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little additional melting occurs at these depths after
this time (Fig. 5B). Temperatures obviously continue to
increase at shallower depths in the mantle after 20 m.y. of
heating but these temperatures never exceed the hydrous
mantle solidus temperatures at these shallower depths,
even when steady-state conditions are achieved (Fig. 5B).
The implication here is that instantaneous conductive
heating from below of hydrous lithospheric mantle
necessarily produces a pulse of magmatism of finite
duration, the length of which is controlled, among other
factors, by the degree of hydration and the thermal
diffusivity of the CLM.
The above considerations suggest that the ignimbrite
flare-up could ultimately be linked to refrigeration and
hydration of the pre-existing mantle lithosphere by
oceanic lithosphere of the Farallon Plate (Fig. 5C).
Removal of this oceanic plate from beneath the southern
Rocky Mountain region in the mid-Tertiary, and conductive heating of the base of the remaining continental
mantle lithosphere by upwelling, normal potential temperature convecting mantle, could then trigger mantle
melting associated with the ignimbrite flare-up (Fig. 5D).
Conductive heating of hydrated mantle initially at
subsolidus temperatures not only provides a mechanism
of more or less simultaneously triggering mantle melting
beneath the entire southern Rocky Mountain region, but
also could account for the ∼20 m.y. life span of the midTertiary magmatism. In fact, with better estimates of
mantle source volumes and duration of magmatism, along
with more rigorous melting and thermal modeling, it may
eventually be possible to estimate the average amount of
hydration of the mantle lithosphere beneath the southern
Rockies. Such an estimate would be based on the fact that
the degree of hydration controls the position in pressure–
temperature space of the hydrous solidus which, in turn,
limits the minimum mantle depth that could reach
temperatures sufficient to trigger melting and, ultimately,
limits the duration of magmatism. We also note that the
low productivity of hydrous melting could have been
essential in restricting the extent of melting in the
lithospheric mantle, allowing the mid-Tertiary magmatism to retain a potassic character that might have been lost
with larger degrees of mantle melting (Asimow and
Langmuir, 2003).
5.2. Sub-lithospheric mantle source
While our crude calculations allow the possibility that
mantle lithosphere was the ultimate source of magmas
parental to the mid-Tertiary magmatism in the southern
Rocky Mountains, even a two-fold increase in the mantle
source volume actually required to fuel this magmatism
would exceed the volume of mantle likely to have undergone partial melting (Fig. 3). If so, decompression
melting of upwelling sub-lithospheric mantle may be
required to produce at least some fraction of the parental
basaltic magmas. In this case, Nd isotopic compositions
of mid-Tertiary basaltic rocks in the southern Rocky
Mountains, previously been interpreted to require a
lithospheric mantle source, may instead represent the
result of cryptic crustal contamination (Glazner and
Farmer, 1992), the product of mixing between high ɛNd
sub-lithospheric mantle-derived magmas and low ɛNd
lithospheric components, and/or the introduction of low
ɛNd crustal components into shallow portions of the
convecting mantle during low angle subduction.
The advantage of decompression melting is that the
volume of mantle needed to supply the mid-Tertiary
magmatism could be induced to melt through the upwelling of mantle directly beneath the major volcanic
centers, without requiring long lateral magma transport
distances in the mantle (Chamberlin et al., 2002). Of
course, if restricted to a ∼ 100 km radius cylinder, then
the ∼ 7 M km3 mantle source volume required for the
SJVF requires that the equivalent of a column of mantle
at least 200 km thick have undergone partial melting
(Fig. 4). Interestingly, estimates of the volume of mantle
that may have been de-densified beneath the SJVF have
been modeled as having similar dimensions (Roy et al.,
2004), although these authors have modeled the affected
mantle as representing the entire thickness of present-day
mantle lithosphere (Dueker et al., 2001). As discussed in
the previous section, conductive heating even of metasomatized mantle could not have induced melting
through the entire thickness of the lithospheric mantle
beneath the SJVF over its ∼ 20 m.y. lifespan. But the dedensified mantle beneath the SJVF, if truly present at
lithospheric, and not sub-lithospheric depths, could
represent the residue remaining of the convecting mantle
that underwent decompression partial melting during the
mid-Tertiary. Such an assertion implies that there must
have been dramatic thinning of the mantle lithosphere
beneath the SJVF both to allow extensive decompression
melting of upwelling convecting mantle (McKenzie and
Bickle, 1988) and to allow this partially melted mantle to
comprise much of the lithospheric mantle today.
We can only speculate as to how lithospheric thinning
could have been accommodated in the absence of obvious extensional tectonism. One option is that the deep,
initially cold portions of the continental lithosphere remaining after the removal of Farallon plate were more
dense than surrounding material and susceptible to removal by gravitational instability. Removal of the deep
lithosphere may not have been an immediate consequence
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G.L. Farmer et al. / Lithos 102 (2008) 279–294
of roll back of the Farallon plate given that, because of the
temperature dependence of viscosity, ductile gravitational
instabilities are inhibited by low temperatures and material cooler than 600 to 700 °C cannot flow sufficiently
fast to form a perturbation and grow into an instability
(Elkins-Tanton, 2007). But progressive heating by upwelling sub-lithospheric mantle could have eventually
triggered removal of the deep lithosphere and its
downward transport into the underlying mantle. During
its descent, this lithosphere could have undergone partial
melting, particularly if hydrated (Elkins-Tanton, 2007).
Mixing between partial melts of the old lithospheric
mantle, with the products of adiabatic melting of upwelling sub-lithospheric mantle provides a method of
producing both the volumes of mid-Tertiary volcanic
rocks and the relatively low ɛNd values of the mid-Tertiary
mafic igneous rocks.
5.3. Sierra Madre Occidental, northern Mexico
Although lithospheric mantle remains a plausible
source for mid-Tertiary magmatism in the southern
Rockies, at least from a mantle source volume perspective, the sheer volume of mid-Tertiary silicic volcanic
rocks further to the south in the Sierra Madre Occidental
(SMO) of northern Mexico (Fig. 1) essentially precludes
the possibility that the CLM could have been the sole
source of basaltic magmas parental to this igneous event.
Silicic magmatism in the SMO occurred in an area similar
to that of the southern Rocky Mountains of Colorado and
New Mexico (∼300,000 km2) in two discrete pulses from
38–28 Ma and 24–21 Ma (Wark et al., 1990; Ferrari et al.,
2002). The total volumes of preserved volcanic rocks in
the SMO are poorly constrained (Swanson et al., 2006),
but assuming an average ignimbrite thickness of ∼1 km
within the SMO proper suggests a total volume of
preserved ignimbrites of ∼392 K km3 , at least six times
the volume of all mid-Tertiary volcanic rocks preserved in
the SJVF (Aguirre-Diaz and Labarthe-Hernandez, 2003).
Including mid-Tertiary ignimbrites from areas peripheral
to the SMO produces a total ignimbrite volume estimate
of ∼587 K km3 for northern Mexico (Aguirre-Diaz and
Labarthe-Hernandez, 2003).
Mantle source volumes calculations for the SMO are
hindered by the lack of information regarding the relative
volumes of andesites, crystal poor rhyolites or crystal-rich
dacites in this region and by the fact that it is not known
whether the mid-Tertiary silicic ignimbrite deposition was
immediately preceded by voluminous intermediate composition volcanism, or whether the entire SMO is underlain by voluminous mid-Tertiary intrusive rocks (Swanson
et al., 2006). There is at least evidence from the northern
291
SMO that mafic to intermediate composition magmas
were involved in the generation of the silicic volcanism
(Wark, 1991). If we simply calculate an “open system”
mantle source volume for the SMO from the ignimbrite
volume estimates alone, assuming the same model
parameters used in Table 1 for the SJVF and MDVF,
and assuming a dacitic composition for the preserved
ignimbrites and a Mint/Mext = 5, then the SMO requires a
mantle source volume of at least 45 M km3. If mantle
melting is restricted to the mantle lithosphere, and a 15 m.y.
duration for the silicic volcanism in the SMO is assumed
(Ferrari et al., 2002), then the reach of the SMO volcanic
centers would have to exceed ∼1000 km (Fig. 4). Such a
wide reach impinges on the mantle source regions for
ignimbrites in the southern Rocky Mountains (Fig. 1) and
requires a maximum lateral transport distance for melt in
the mantle more than three times that required for the SJVF
over a similar duration of magmatism. We consider these
requirements as strong evidences against a lithospheric
mantle source for basaltic magmas parental to the SMO, so
indirectly supporting models in which the SMO magmatism is produced through dynamic melting of sublithospheric mantle (Ferrari et al., 2002).
6. Conclusions
Crude estimates of the volume of upper mantle needed
to supply the mass of basaltic magma involved in the midTertiary ignimbrite flare-up in the southern Rocky
Mountains suggest these magmas could have been
generated by conductive heating of continental lithospheric mantle, but only if the base of the lithosphere throughout
the entire region underwent partial melting. Widespread
melting of the CLM in the mid-Tertiary is plausible in the
southern Rocky Mountains if the CLM were metasomatized in the early Tertiary by fluids derived from the
dehydration of underlying, shallowly subducting oceanic
lithosphere. Melting could then have been triggered by
removal of the oceanic lithosphere in the mid-Tertiary and
re-exposure of the base of the mantle lithosphere to
convecting mantle, even if the upwelling convecting
mantle was at “normal” potential temperatures. Such a
conductive heating model requires focusing of mantle
melt, over distances of over 300 km, into one of only two
major “drains” through the continental lithosphere, now
represented at the surface by the San Juan and MogollonDatil volcanic fields. While conductive melting of the
CLM is a possible, but not required, method of fueling
mid-Tertiary magmatism in the southern Rocky Mountains, further south in the Sierra Madre Occidental the
volumes of silicic magmatism are too large to be supported
solely by melting of CLM. Here, melting in the sub-
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lithospheric mantle must have played a significant role in
producing the mid-Tertiary igneous activity.
Acknowledgements
This paper benefited greatly from discussions with
Craig Jones, Gene Humphreys, Eric Christiansen, Allen
Glazner, and Doug Walker, and from the journal reviewers, but we take full responsibility for its contents. The
work was supported by NSF grants EAR-0112673 and
EAR-0313319 (to Farmer).
References
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