Download Zircon U-Pb grain ages clustering at several ages between 92 and

The web of pegmatitic
leucogranite gneiss inflates an
undisrupted ghost stratigraphy
of metamorphosed
Neoproterozoic to Devonian
carbonate and quartzite strata.
Pegmatitic
leucogranite makes up
nearly half of the
rocks within the
mapped stratigraphic
units.
Emplacement environment for this
peraluminous granite was an estimated 6–7 kb,
>500°C.
Compositions of the pegmatitic granite are
consistent with derivation from muscovitedehydration melting of underlying Neoproterozoic
pelitic rocks (Lee et al., 2003).
Adjacent country-rock strata and their
protoliths --metacarbonate rocks and quartzite,
minor pelite--were not sources.
The Zircons are Unusual
• Mottled grain interiors
+ oscillatory zones.
• Low T (600–700°).
• Very high U (1000-15,000
ppm).
• Very low Th/U (<0.01).
• Hi U/Ce ratios.
• Very high Hf, 13k-26k ppm.
• Heavy-REE depleted.
• Deep negative Eu anomalies.
113
92 Ma
Multiple
zircon ages
between and
within
samples.
37
67
~40
~29
Precambrian
xenocrysts,
present in
some other
J, K & T
granites in the
area, are
absent from
the pegmatitic
granite.
92 Ma
37
67
~40
~29
92 Ma
37
Instead we
have
antecrysts ??
(sort of).
67
~40
~29
SHRIMP UPb grain dates
• Zircon U-Pb grain ages clustering at several ages between 92 and
68 Ma and between 42 Ma and 30 Ma indicate successive or
rejuvenated igneous zircon growth multiple times over long
periods of Late Cretaceous time and late Eocene to Oligocene
times.
We interpret
clusters of
zircon ages as
times of new
zircon growth
in partial
melt.
Possible explanations of bimodal or
multiple age ranges within and
between samples:
• Metamorphic
overprints or Pb loss.
Possible explanations of bimodal or
multiple age ranges within and
between samples:
• Metamorphic
overprints or Pb loss.
• Composite ages
across crystal zones.
Possible explanations of bimodal or
multiple age ranges within and
between samples:
• Metamorphic
overprints or Pb loss.
• Composite ages
across crystal zones.
• Samples are
composites across
heterogeneous
intrusions.
Possible explanations of bimodal or
multiple age ranges within and
between samples:
• Metamorphic
overprints or Pb loss.
• Composite ages
across crystal zones.
• Samples are
composites across
heterogeneous
intrusions.
• Inherited grains
present.
Possible explanations of bimodal or
multiple age ranges within and
between samples:
• Metamorphic
overprints or Pb loss.
• Composite ages
across crystal zones.
• Samples are
composites across
heterogeneous
intrusions.
• Inherited grains
present.
• Partial
remobilization and
recrystallization in
situ.
Pegmatitic granite
forms networks of
sills, sheets, and
dikes.
Coalesced pegmatitic granite intrusions
intricately inject calc-silicate host rocks,
isolating remnants of the metasediments.
Host-rock remnants define undisrupted ghost stratigraphy
and structure.
Sheets of the white granite are more numerous low on canyon walls,
including on the lower limb of a large fold nappe.
The increased percentage of granite at lower structural levels is
interpreted to reflect more advanced stages of incremental
assembly of the batholith.
Based on the ghost stratigraphy and intrusive
style, the batholith was assembled
incrementally by myriads of small intrusions.
As the intrusions coalesced, boundaries
became blurred.
Do the multiple U-Pb ages represent new
magma injections from depth?
Or could they represent slight remobilizations at
or near the observed site of intrusion?
Folded sills that merge with cross-cutting dikes
are consistent with remobilization of preexisting intrusive bodies.
SUMMARY
Low-temperature hydrous granites were
emplaced incrementally into host rocks without major disruption of
host-rock structure.
Zircons show large age ranges suggestive of multiple episodes
of remobilization in partial melts. Fluxes of hydrous fluids?
These processes extended from ~93 Ma to 68 Ma and ~40Ma to
30 Ma.
Intrusion style and SHRIMP geochronology for this plutonic
complex in the Ruby Mountains core complex suggest very
lengthy construction and reconstruction of a batholith-sized
volume by incremental coalescence of small intrusions.