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PETROGENESIS AND MANTLE DYNAMICS OF PALEOZOIC VOLCANISM IN
THE SÔNG ĐÀ STRUCTURE
Nguyễn Hoàng1*, Nguyễn Đắc Lư2, Nguyễn Văn Can2
1. Institute of Geological Sciences, Láng Trung, Hà Nội
2. Northern Geological Mapping Division, Long Biên, Hà Nội
(*) Now at Geological Survey of Japan, Higashi 1-1-1, Central 7th, Tsukuba, JAPAN
305-8567; E-mail: [email protected]
Abstract
Paleozoic volcanic activity in association with lithospheric extension and formation of the
Song Da Structure produced mainly alkalic and sub-alkalic basalts with a subsidiary amount of
rhyolite, trachyte and transitional types. The basalts are distinguished by high- and low-Ti.
High-Ti (TiO2 > 1wt%) type is enriched in highly incompatible as well as light rare earth
elements and is also high in FeO* and ratios such as Zr/Y, Nb/Y than the companying low-Ti
type (TiO2 < 1%) is interpreted to be result of melting from fertile asthenospheric material with
involvement of mafic veins such as clinopyroxenite and amphibolite, etc. Whereas, the low-Ti
type may be product of melting of the fertile and enriched asthenosphere mixing with
refractory and depleted lithospheric mantle as the first rose following lithospheric extension
and erosion.
Initial 87Sr/86Sr, 143Nd/144Nd and 206Pb/204Pb isotopic compositions were obtained based on
reported age of 283 Ma ranging from 0.7036 – 0.7090, 0.5119 – 0.5124, and 18.32 – 23.50,
respectively. Many of the values fall in enriched fields relative to CHUR calculated for 283
Ma (87Sr/86Sr = 0.7041 and 143Nd/144Nd = 0.5123). Isotopic enrichment accompanied by
negative anomalies at Nb, Ta, and Zr was interpreted to be derived from mantle source
contaminated by crustal material introduced via plate subduction. We suggest two dynamic
models that might be driving force for the volcanic activity, (1) intraplate lithospheric
extension, resulted from long-term plate compression; (2) back-arc spreading at a plate
margin. Such the mantle-lithosphere interaction dynamics should eventually lead to
progressive melting without the necessity for a mantle plume to present.
I. Introduction
For many years the Sông Đà structure has been a subject of intensive study and debate on
its geodynamics and related volcanism. Some viewed the structure as a Triassic or
Mesozoic depression (or rift) [7-9, 38], others believed it was a geosyncline with oceanic
ophiolite complexes [40]. However, to date, many researchers have come to an agreement,
although with no less controversy, that the structure is an inner-continent rifted zone,
started possibly from early Permian and lasted until late Triassic as a result of a prolong
compression process related to regional deep faults [7-9, 38].
Paleozoic volcanism occurred widely within the Sông Đà structure zone (CTSD) at Suối
Chát, Kim Bôi, Viên Nam, Ba Vì, ect. (Fig.1). The products are mostly basalt together
with a subsidiary amount of intermediate and acidic types such as andesite, trachyte and
rhyolite. Many detailed studies of the volcanic rocks were conducted over years [3, 7, 8,
31, 36, 37]. Aside from the complexity in temporal and spatial relationships among the
lavas, there is a commonly reported feature that low- and high-titanium (lo-, hi-Ti) basalt
types are recognized within the rift zone regardless of spatial distribution [36, 37, and
references therein]. The hi-Ti basalt type, in contrast to the low-Ti type, has higher total
alkalis and lower MgO contents. Meanwhile, for some lo-Ti basalts having very high MgO
are termed as komatiite [3], which is normally found in Archean lithosphere [2].
Age of the volcanic eruption is controversial. Radiometric age data are scarce, but Balykin
et al [3] reported a Rb-Sr age acquired on clinopyroxene separates (?) from a komatiitebasalt suite in the northwestern side of the rift zone to be 257 ±7.2 Ma (early Permian)
with an initial 87Sr/86Sr of 0.70299 ±3. Hoang et al [27] reported an Rb-Sr age of 283 ±21
Ma and 87Sr/86Sr (initial) at 0.70667 ±0.000056 for a set of samples ranging from basalt to
trachy-rhyolite in the Đồi Bù - Suối Chát area. The latter age is in accordance with late
Carboniferrous fossils uncovered in limestone lenses embedded within basaltic layers in
the Hoà Bình- Suối Rút area [32].
We collected a set of samples represented for various rock types in different areas within
the structure (Fig. 1) to analyze for chemical and isotopic compositions. The data are
reported to discuss the petrogenesis, mantle source, possible involvement of the
lithospheric mantle in the context of regional geodynamics.
II. Geology and Petrography
Volcanic rocks range from mafic to acidic in compositions and are divided into two
eruptive phases. The first phase includes mafic (ca. 80%) and sedimentary volcaniclastic
products. They are thick lava flows of picro-basalt, basalt, andesitic basalt together with
thin lenses of basaltic tuff outcropped in the areas of Suối Chát, Viên Nam, Ba Vì and Đồi
Bù. Most basalt layers are greenish due to weathering (chloritization and epidotization).
Besides, there are a number of sub-extrusive diabase bodies with thickness ranging from 5
to 10 meters, in some case up to 30 meters, cutting through earlier eruptives. Second phase
is represented by lavas of intermediate to acidic types such as trachyte, andesite, dacite
and rhyolite.
Basalts are mostly aphyritic. Porphyritic type has olivine, clinopyroxene and plagioclase in
the phenocryst (up to 15 vol.%). Many of the phenocrysts, however, are altered. For
example, plagioclase is mostly sericitized, olivine becomes iddingsite and clinopyroxene
is chloritized or epidotized. Basalt textures are microdoleritic or intersertal, whereas
structures are massive to porous with pores filled with chlorite, epidote or K-feldspar.
Trachyte and trachy-rhyolite types are porphyritic with idiomorphic plagioclase, Kfeldspar and albite in the phenocryst (up to 25 vol.%). The groundmass contains Kfeldspar, amorphous quartz and volcanic glass. Phenocrysts of porphyritic rhyolite are
quartz and K-feldspar (up to 15 vol.%), and groundmass contains micro-quartz and Kfeldspar aggregates, the latter are partly kaolinized.
III. Analytical Procedure
Major and some trace elements were obtained in Vietnam using an X-ray Fluorescence
spectrometer; whereas rare earths were obtained using instrumental neutron activation
analysis (INAA). Isotopic analyses were conducted at the Geological Survey of Japan
(GSJ). Detailed analytical procedure, accuracy and precision are given in [28]. Data are
shown in Table 1.
For Sr, Nd and Pb isotope analysis, only fresh rock chips were crushed to pieces of 1-2
mm in size. The chips were warmed in HCl 3N in 15 ml Teflon beakers for about 30
minutes then washed ultrasonically in the acid for about an hour, followed by multiple
rinses with clean water before being ground in an agate mill. About 50 mg of the powder
was dissolved in concentrated HNO3 and HF, repeated with HNO3. All the acids used
were certified ultra-clean grade. Elemental extractions were described in [28]. Rb, Sr, Nd
and Pb isotope ratios were measured on a multi-collector VG Sector 54 thermal ionization
mass spectrometer at GSJ. The 87Sr/86Sr was normalized to 86Sr/88Sr = 0.1194 and the
143
Nd/144Nd was normalized to 146Nd/144Nd = 0.7219. The within-run precision (2) for
87
Sr/86Sr was 0.000006 to 0.000009 and 0.000007 to 0.000012 for 143Nd/144Nd.
During the period of measurement 87Sr/86Sr of the NBS 987 Sr standard was 0.710265
0.000008 (1, n = 28) and 143Nd/144Nd for the JNdi-1 (GSJ) Nd standard [see 20] was
0.512105 0.000005 (1, n = 19). Lead isotopic compositions were corrected for mass
fractionation, and are reported relative to the NBS 981 Pb standard values of (mean, 1, n
= 26) 36.564 0.020, 15.453 0.010, 16.908 0.008 for 208Pb/204Pb, 207Pb/204Pb, and
206
Pb/204Pb, respectively. Internal precision of the Pb ratios (2) is less than 0.01% and
total blank is less than 50 pg. The data are shown in Table 2.
IV. Analytical Results
1. Major element compositions
Based on the analytical results the lavas may be divided into two chemical groups, the first
with SiO2 ranging from 59 to 70 (wt%) includes trachyte, andesite, dacite and rhyolite. In
this report this group is termed as felsic. The second having SiO2 contents ranging from 44
to 50 (wt%) is termed as mafic. However, because of the large difference in TiO2 contents
among the mafic types, following previous researchers [36, 37], we divided the group into
low- (TiO2 < 1%) and high-titanium (TiO2 > 1 %) sub-groups (Table 1, Fig. 2). While the
felsic type concentrates in the fields of trachyte, trachy-andesite, and rhyolite, hi-Ti mafic
type having higher total alkalis (up to 6 wt%) than its low-Ti counterpart falls within the
alkaline field (Fig. 2). Moreover, hi-Ti rock type has higher FeO (average 12.5 wt%
compared to 8.7 wt%) and lower MgO (average 5.6 wt% compared to 13.6 wt% in the
low-Ti type) at a similar SiO2 content (Table 1). A low-Ti sample that falls in the picrobasalt field has MgO = 24.5 %, Ni = 1297 ppm and Cr = 2922; however FeO is much too
low (ca. 9.5 wt%) to be considered as komatiite [2, cf. 3, 37]. We, therefore, classify the
rock as a picro-basalt that belongs to the low-Ti mafic type.
2. Trace element chemistry
The felsic samples have high incompatible elements (Rb, K, Th) and rare earth elements
(REE), however, they show relatively low contents of high-field strength elements (HFSE)
(Table 1, Fig. 3). Samples of hi-Ti mafic group show much higher incompatible elements
compared to those of the lo-Ti group. For example, both La and Yb are high in hi-Ti
samples, besides, La contents are too high relative to Yb leading to La/Yb ratios are much
higher than in the lo-Ti samples (Table 1). In general, hi-Ti samples are highly enriched in
light REE relative to heavy REE. This feature is easily recognized in Figures 3b-c,
expressed by steeper angle from La down to Yb in hi-Ti samples compared to lo-Ti
samples. In addition, similar to that observed for the felsic group, both hi- and lo-Ti
samples show relatively strong negative anomaly at some HFSE, especially, Nb and Zr.
Aside from the negative anomaly configurations of trace element patterns of the mafic
samples are much similar to those of intraplate basalts (for example, oceanic island basalt:
OIB) and different from mid-ocean ridge basalt (MORB) or arc lavas [12, 17, 33].
3. Isotopic compositions
Isotopic compositions were corrected for initial values at 283 Ma [27] and shown in Table
2. Corrected results show that initial strontium isotopic ratios vary in a much narrower
range from 0.7055 to 0.7065, accompanied by 143Nd/144Ndi at 0.5124 to 0.5123 and
208
Pb/204Pbi, 207Pb/204Pbi, 206Pb/204Pbi, respectively, within 39.43 – 46.91, 15.66 – 15.92,
18.91 – 23.57 (Table 2). Strontium and lead isotopic ratios (initial) of lo-Ti samples are
lower, and 143Nd/144Ndi ratios are higher than hi-Ti samples. A lo-Ti sample from Bản
Tăng shows 87Sr/86Sri at 0.7036 and 143Nd/144Ndi at 0.51236, falling in the Depleted Mantle
field (at 283 Ma). Correlation between Sr and Nd isotopes develops into two trends. One
negative covariance, connecting depleted mantle field with enriched continental crust, the
second is a nearly straight line where Nd isotopes are nearly constant over a large range of
strontium isotopes (Fig. 4a). Lead isotopes meanwhile show positive correlation of
206
Pb/204Pbi against 207Pb/204Pbi and 208Pb/204Pbi reflecting uniformly isotopic integration
among related U, Th and Pb isotopes. Except for a felsic sample that has very high lead
isotopic ratios and plots in a separate field, apart from the field for the rest of the samples
(Figs. 4b-c).
V. Discussion
As described above Paleozoic volcanic rocks in the Song Da Structure have undergone
post-melting modification processes, including fractional crystallization and weathering
alteration. Fractional crystallization is evident by low MgO, average about 5.5 (wt%),
much lower than would-be primitive composition [13, 15, 19]. Also, negative anomaly at
Sr may reflect fractionation of plagioclase (Fig. 3a). Evidence of the volcanic products
having undergone weathering alteration includes primary phenocrysts replaced by
secondary minerals. For example, olivine is replaced by iddingsite; clinopyroxene is
chloritized or epidotized; while plagioclase is vastly sericitized. Under the impact of
alteration processes elements such as Rb, K, Na can be highly mobilized compared to
HFSE such as Ti, Zr, Y and Nb [29].
1. Crustal contamination
Magma passing through or incubating in the crust may interact with surrounding material.
Crustal contamination of mafic magmas is expressed by having low Nb, Ta, Nb/Y and
Ta/Y, and high concentrations of incompatible elements such as Rb, Ba, K, relative to
other trace elements [6, 12, 16]. Negative anomalies at HFSE suggest that volcanic rocks
in the Song Da Structure might be contaminated. Crustal contamination is resulted in
having high strontium and low neodymium isotopic ratios that head toward the continental
crust field (Fig. 4a). Except for a sample with low Sr and high Nd isotope that falls in the
depleted field all the rest of the samples distribute in enriched fields relative to Chondrite
calculated for 283 Ma (CHUR283 Ma : 87Sr/86Sr = 0.7041, 143Nd/144Nd = 0.5123). In short,
most of the studied samples were contaminated by crustal material either after or before
melting (products of contaminated source mantle).
Lead concentration in the crust is high but Nd isotope is low. Interaction between mantlederived magmas with crustal material results in positive correlation between Nd isotopes
and Ce/Pb ratios as illustrated in Figure 5a [4, 5]. Similarly, effect of crustal contamination
may reflect in negative covariance between Sr isotopes and Nb/Y ratios; however, the
correlation is positive for most of the samples (Fig. 5b). Therefore, timing of the crustal
contamination is inconclusive. Negative and positive correlation between, respectively, Nd
and Sr isotopes, and Ce/Pb (Figs. 4a and 5a) may reflect mixing between an enriched and
a depleted source. For example, an alkaline rhyolite (Đồi Bù, VD502), traditionally
viewed as a crust-derived volcanic rock for usually having low Nd isotope and Ce/Pb (or
high Sr isotope and low Nb/Y), shows high Nd isotope and Ce/Pb, plotting closer to the
depleted field (Fig. 5b). Another example, sample VD3094 (lo-Ti basalt, Bản Tăng) while
having high Nd and low Sr isotope distributing within the depleted field (Fig. 4a) has low,
crustal-like Ce/Pb ratio (Fig. 5a). Therefore, effect of crustal contamination is inconclusive.
Instead, trace element and isotopic characteristics of Permian volcanic rocks in the Song
Da structure may possibly reflect mixing between depleted and enriched sources.
2. Melting process
Correlation between TiO2/Al2O3 and TiO2 is not affected by fractionation of olivine and
clinopyroxene. Besides, ratios such as Ti/Y, Zr/Y and Nb/Y may be used as indicators to
evaluate source mantle and melting process because they are not affected by fractional
crystallization and relatively resistant to alteration processes [28, 29, 39]. However, in
order to understand mantle source region and melt segregation conditions it is essential
that chemical compositions of the primitive melt be known.
Volcanic rocks in the Song Da Structure have high contents of incompatible elements; and
high light-REE relative to middle and heavy REE (Table 1, Figs. 3a-c). Elements such as
Sm, Yb and Y are higher in the hi-Ti samples but much lower in the lo-Ti lavas relative to
MORB [e.g. 14, 15, 33]. The elements are highly compatible in garnet-bearing rocks while
incompatible in the presence of other minerals, such as spinel [15, 18, 24]. Mid-ocean
ridge basalts are believed to be derived by melting of refractory spinel peridotite that
underwent previous melting events occurred during processes of oceanic crust formation
[15, 23, 39, and references therein]. Very lower concentrations of Sm, Yb and Y in the loTi mafic samples compared to MORB may suggest that garnet was a residual mineral. In
contrast, high contents of the elements in the hi-Ti mafic samples may indicate the lavas
were products of spinel peridotite melting. Moreover, higher ratios of La/Yb in the hi-Ti
samples relative to lo-Ti samples may suggest that they were derived from a more
enriched source and/or smaller melting degrees [15, 18, 23, 33]. In short, distribution
configuration of rare earth pattern not only reflects geochemical characteristics of source
region but also melt segregation condition and nature of primary magma.
Chemical compositions of primary magmas in the Song Da Structure are unknown.
However, they can be approached by addition (or extraction) of olivine (and
clinopyroxene) that crystallized (or cumulated) during the process of magma evolution to
the present composition [29, 41]. As described above, many of the Song Da volcanic rocks
have olivine and a minor amount of clinopyroxene in the phenocryst. Besides, some
cumulate olivine aggregates were observed in lo-Ti samples [3, 36, 37]. Therefore, we
may conclude that most of the mafic rocks underwent olivine fractionation at early stages
and olivine + clinopyroxene at later stages before the magmas erupted to the surface. In
order to simplify the calculation task while protecting the meaning of searched primary
chemistries, in this report we conduct only olivine correction. The calibration is based on
the following criteria: first, primary mafic melt, product of spinel or garnet peridotite
melting, has a Mg-number [100*Mg/(Mg + Fe2+)] in the range of 69 to 71 [13, 18, 19, 41];
second, olivine - melt Fe2+/Mg distribution coefficient (Kd Fe++/Mg (Ol/liq) = 0.30 [34]) and
composition of olivine is Fo87 to Fo89; third, in order to avoid plagioclase fractionation
effect only samples with MgO higher than 6 (wt%) were applied for correction [39].
Based on the above criteria we added Fo85 olivine step by step to mafic rocks at the ratio
of 1:99 until the rock reached a Mg-number between 69 to 70.5 and correspondent olivine
fell within Fo87 to Fo89. In short, depending on chemical composition, olivine was added
(+) or subtracted (-) [41]. Results are shown in Table 3. Note that although the calculated
results might not reflect the true chemistry of primary melts they show, however, that
fractional crystallization of olivine (and clinopyroxene) increases TiO2 in the melt while
TiO2/Al2O3 ratios are nearly unchanged. As illustrated in Figure 6 that high TiO2 contents
in hi-Ti samples are not derived by melting of either spinel or garnet peridotite (line 2 and
3). Results of experimental melting of peridotite shows that the lower melting degree the
higher Ti content in the melt, and that at the same melting parameters the more fertile
peridotite produces melt with higher Ti (line 2 compared to line 3 in Figure 6). In addition,
with melting degree smaller than 1%, melting of a fertile peridotite produces melt with
maximal TiO2 1.5 (wt%) and corresponding TiO2/Al2O3 < 0.1 [9, 15, 23]. Obviously, the
hi-Ti mafic rocks, unlike their companying lo-Ti, must be derived from a much higher Ti
source. High Ti mantle-derived rocks may include clinopyroxenite, wehrlite, websterite,
amphibolite, etc. They usually have high TiO2, Al2O3, TiO2/Al2O3 and incompatible
elements compared to lherzolite or harzburgite. They normally include high-Ti
clinopyroxene and amphibole (kaersutite), ± phlogopite and secondary minerals such as
apatite, rutile, and ilmenite [5, 24]. Experimental melting of mixture of a peridotite and a
composite basalt component by Kogiso and colleagues [18] produced melts with high FeO,
TiO2, and TiO2/Al2O3, similar to the hi-Ti mafic samples reported here (line 1 in Fig. 6).
This observation suggests involvement of mafic component(s) in the petrogenesis of hi-Ti
mafic lavas in the Song Da rift zone.
Hi-Ti lavas have much higher Zr/Y, Ti/Y and Nb/Y compared to lo-Ti samples (Figs. 7a, b)
Because the correlation between Nb/Y and Zr/Y (Ti/Y) is not effected by low-pressure
fractional crystallization (pyroxene ± plagioclase) difference in these ratios in lo- and hi-Ti
reflects melting degree, depth of melt segregation and source fertility [13, 23]. Besides,
melt-solid distribution coefficients (Kd liq/solid) of Nb <Zr <Ti <Y [5, 14, 16] high ratios of
Nb/Y, Zr/Y and Ti/Y in hi-Ti lavas may suggest that they were derived by lower melting
degree and/or from a more fertile source relative to lo-Ti magmas.
3. Source characteristics and geodynamics of the Song Da volcanism
Above we have shown that melting of a mixture of peridotite and mafic component may
produce melts with high Ti, Fe and incompatible elements. Distribution of mafic veins in
ultramafic bodies is common [30]. These ultramafic bodies are residues, cumulated after
melting of asthenosphere to be the major component of lithospheric mantle. The existence
of mafic veins is explained as small volumes of trapped basaltic melt, products of local,
discrete melting within the lithospheric mantle [6], or products of metasomatic processes
under the influence of asthenospheric heat [24, 25]. In general, the lithospheric mantle is
composed mostly of refractory peridotite rocks. They are residues after melting of
asthenospheric material to form mafic melts, therefore, they are enriched in Mg and
depleted in basaltic components such as Fe, Ti, Ca, Al, Na, etc., light-REE and especially,
very depleted in highly incompatible elements [1]. However, due to metasomatic activities,
interaction with mafic melts introduced from below, trace element budget in the
lithospheric mantle may be increased [1, 6, 24, 25]. This re-enrichment process is
obviously depended on geochemical environment and duration. In summary, geochemistry
of the lithospheric mantle is considered to be complicatedly heterogeneous, and the
heterogeneity can be in a large as well as small scale [25]. In contrast, asthenosphere
material is believed to be homogenous, fertile in basaltic component, and enriched in trace
element concentration [1].
The Song Da structure has been viewed as a depression (rift) that developed from late
Permian till late Triassic [7-9, 31, 38]. Regardless of whether the rift initiated in an
intraplate or marginal environment, during lithospheric subsidence and extension
processes, base of the lithospheric mantle will rise and be eroded under heat impact from
the asthenosphere that rises up to fill in gaps left behind by lithospheric mantle in
accordance with the uniform stretching model [20, 23]. This interaction leads not only to
discrete metasomatism and localized melting to develop within the lithospheric mantle but
eventually ignites decompression melting of the asthenosphere [1, 15, 23]. In this case,
mixing between asthenospheric melts with mafic veins should generate hi-Ti volcanic
rocks with high Fe and trace elements as we argued above. Whereas, melting of the
asthenosphere with a contribution of (garnet-bearing peridotite) lithospheric mantle may
produce melts with low Fe/Mg, relatively low trace element concentrations similar to the
lo-Ti mafic rocks [1, 24, 33, 39]. Difference in the chemical compositions between lo- and
hi-Ti mafic types may reflect the difference in participating ratio between mafic veins
and/or lithospheric mantle versus asthenospheric material, and degrees of partial melting.
Involvement of mafic components and lithospheric mantle has been invoked to explain the
formation of lo- and hi-Ti volcanic rocks in the Oslo rift [26] and the Paranã volcanic
province [22]. Participation of clinopyroxenite veins in the melting of asthenosphere is
viewed to be essential in the formation of Hawaiian high Ti alkaline basalts [18].
Lithospheric mantle is considered to be chemically heterogeneous, ‘cold’, ‘dry’, depleted
in basaltic component, and low in viscosity [1]. Therefore, it has lower chance of melting
to form large volumes of basaltic melts at rifted zones. However, there are opinions that
lithospheric mantle can melt to produce basalts if it becomes more viscous [29, 39].
Dehydration of hydrous minerals such as phlogopite, amphibole, etc., may lead to
decrease solidus temperature, on the one hand, and raise the viscosity, on the other. In both
cases, physical properties of the lithospheric mantle become more asthenospheric, which
are more ready to melting [29, 39].
For a long time the Song Da structure has been coined as a Permian – Triassic ‘innercontinental’ rift (depression). We believe ‘continental’ may cause confusion since during
the Permian, the territory was possibly oceanic [10, 11, 32, 35]. The volcanism was not an
island arc related for there is very high percentage of mafic rocks (80%) over intermediate
and acidic types. There is no evidence of being mid-ocean ridge volcanism for the latter
has no acidic volcanic lavas. Therefore, the environment for the Song Da volcanism to
appear was most likely intraplate or marginal with an oceanic (not continental) lithosphere
that was composed of mafic and ultramafic layers. This argument points out that the
crustal contamination discussed above could not have happened during the magma
passage to the surface but rather it may happen in the mantle before the melting occurred.
Crustal material, especially hydrous minerals in oceanic sediments, may be introduced into
the mantle at subduction zones. Under high temperature and pressure condition hydrous
minerals dehydrate to form hydrous fluids that carry highly soluble elements such as Ba,
Rb, K, Th, etc., to shallower mantle, leaving behind poorly soluble HFSE such as Ti, Nb,
Zr, etc.[5, 12, 16, 17]. Melting of mantle mixing with lesser than 1% of crustal material
produces anomalies not only in Sr, Nd and Pb isotopic compositions but also in the
configuration of trace element patterns [see 28 for calculation example]. Mantle may be
contaminated by crustal material in global scale (for example, Indian Ocean asthenosphere
[21]), or regional scale, for example, mantle in the proximity of west Pacific subduction
zone [12, 17, 41]. Ancient subduction zones had widely developed behind Eurasia in
accordance with the development and disappearance of the Paleo-Tethys sea (ca. 400 Ma
to 200 Ma [35]). Crustal contamination of source region of the Song Da volcanism may be
regional that developed over a long period, or it may reflect a local phenomenon for being
in proximity to a subduction zone.
From the above discussion we present two geodynamic models for the Song Da volcanism,
1) intraplate lithospheric extension, and 2) back-arc spreading resulted from subduction.
The first model may be viewed as a result of prolong compression related to regional deep
fault systems (see above). For the second, extension occurred following erosion of
lithospheric base by mantle convection caused by a subduction zone. However, the second
model requires that the Song Da Structure be located at an active margin.
VI. Conclusions
1. Permian volcanism in the Song Da Structure consists mainly of mafic rocks with a
subsidiary amount of intermediate and acidic lavas. The mafic type is classified
into high-Ti (TiO2 > 1 wt%) and low-Ti (TiO2 < 1 wt%). High-Ti samples have
higher FeO*, total alkalis and concentrations of trace elements than low-Ti lavas.
2. High-Ti magmas are explained by melting of fertile asthenosphere mixing with
mafic veins in the lithospheric mantle enriched in Ti and incompatible elements.
Whereas lo-Ti rocks are explained by mixing between fertile asthenosphere and
refractory lithospheric mantle melts.
3. High Sr, Pb and low Nd initial (283 Ma) isotopic compositions together with
negative anomalies at high-field strength elements (Nb, Ta, Zr, Y) suggested that
source mantle might be contaminated by crustal material introduced into the
mantle via a subduction zone.
4. Two dynamic models might be driving force for the volcanic activity, (1) intraplate
lithospheric extension, resulted from prolonged plate compression; (2) back-arc
spreading at a plate margin. Both of the dynamics might be related to the
development and disappearance of Paleo-Tethys (ca. 400 Ma to 200 Ma).
5. Mantle-lithosphere interaction dynamics provided by the models should eventually
lead to progressive melting to form the volcanic rocks without the necessity for a
mantle plume to present.
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Figure captions
Fig. 1: Distribution scheme of Paleozoic volcanic localities in the research area. (a) mafic
and sedimentary volcaniclastic products, (b) acidic products.
Fig. 2: Classification of volcanic rocks in terms of SiO2 vs. total alkalis. Note most of the
hi-Ti mafic rocks (circles) plot in alkaline field, whereas lo-Ti type (diamonds) plots in
sub-alkaline field. Crosses are felsic rocks.
Fig. 3: Incompatible trace element distribution normalized to primitive mantle for (a)
felsic, (b) low-Ti mafic and (c) high-Ti mafic rocks. Also shown is N-MORB
representative for comparison. Negative anomalies at Sr may reflect plagioclase
fractionation. Normalizing data from [14]. See text for details.
Fig. 4: Correlation between initial isotopic compositions of Sr and Nd (a), 206Pb/204Pb vs.
207
Pb/204Pb (b), and 208Pb/204Pb (c). Initial compositions were calculated based on 283 Ma
age for the lavas [27]. Depleted mantle (DM) and enriched continental crust (CC) relative
to CHUR283Ma. See text for explanations.
Fig. 5: Plots of 143Nd/144Ndi vs. Ce/Pb (a) suggest possible crustal contamination for some
Sông Đà volcanic rocks contradicting to broadly positive correlation between 87Sr/86Sri vs.
Nb/Y (b). Labels as in Fig. 3.
Fig. 6: Relationship between primitive TiO2/Al2O3 vs. TiO2 for lo- and hi-Ti Sông Đà
mafic rocks (Table 3). Results of experimental melting [13] for refractory (line 3), fertile
peridotite (line 2) and mixed peridotite – mafic component (line 1) [18], arrows point in
the directions of progressive partial melting. Distribution field of average world-wide
basalts (contoured) is shown for comparison. Note that fractionation of olivine and
clinopyroxene does not affect TiO2/Al2O3 ratios.
Fig. 7: Correlation of Nb/Y vs. Zr/Y for Sông Đà Paleozoic lo- and hi-Ti mafic rock types
relative to fields of mantle-derived primitive basalts (dashed lines) and depleted MORB.
The higher Nb/Y and Zr/Y the more enriched source and/or the lower melting degree. See
text for details. Labels as in Fig. 3.