New Articles for Geosphere Posted Online in December
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations studied this month include the
North Cascades, USA; the Wrangell Arc; the South Mountain Batholith,
Canada; and the Adirondack Highlands, USA. You can find these articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Multiple sediment incorporation events in a continental magmatic arc:
Insight from the metasedimentary rocks of the northern North Cascades,
Washington (USA)
Ann E. H. Hanson; Stacia M. Gordon; Kyle T. Ashley; Robert B. Miller;
Elizabeth Langdon-Lassagne
Abstract
:
The rheology and composition of arc crust and the overall evolution of
continental magmatic arcs can be affected by sediment incorporation events.
The exhumed Cretaceous–Eocene North Cascades arc exposes abundant
metasedimentary rocks that were incorporated into the arc during multiple
events. This study uses field relationships, detrital zircon geochronology,
bulk rock geochemistry, geothermometry, and quartzingarnet geobarometry
to distinguish approximate contacts and emplacement depths for different
metasedimentary units to better understand their protolith incorporation
history and impact on the arc. The Skagit Gneiss Complex is one of the main
deep crustal units of the North Cascades arc. It includes metasedimentary
rocks with distinct detrital zircon signatures: Proterozoic–Cretaceous
(Group 1) or Triassic–Cretaceous (Group 2) zircon populations. Both
metasedimentary groups achieved near peak metamorphic conditions of
640–800 °C and 5.5–7.9 kbar; several Group 2 samples reveal the higher
pressures. A third group of metasedimentary rocks, which was previously
interpreted as metamorphosed equivalents of backarc sediments (Group 3),
exhibited unimodal Triassic or bimodal Late Jurassic–Early Cretaceous
detrital zircon signatures and achieved nearpeak conditions of 570–700 °C
and 8.7–10.5 kbar. The combined field and analytical data indicate that
protoliths of Group 1 and Group 2 metasedimentary rocks were successively
deposited in a forearc basin and underthrusted into the arc as a relatively
coherent body. Group 3 backarc sediments were incorporated into the arc
along a transpressional stepover zone. The incorporation of both forearc
and backarc sediments was likely facilitated by arc magmatism that weakened
arc crust in combination with regional transpression.
View article
:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02425.1/610424/Multiple-sediment-incorporation-events-in-a
Geochronology of the Wrangell Arc: Spatial-temporal evolution of
slab-edge magmatism along a flat-slab, subduction-transform transition,
Alaska-Yukon
Jeffrey M. Trop; Jeff A. Benowitz; Carl S. Kirby; Matthew E. Brueseke
Abstract
:
The Wrangell Arc in Alaska (USA) and adjacent volcanic fields in the Yukon
provide a long-term record of interrelations between flat-slab subduction
of the Yakutat microplate, strike-slip translation along the
Denali–Totschunda–Duke River fault system, and magmatism focused within and
proximal to a Cretaceous suture zone. Detrital zircon (DZ) U-Pb (n = 2640)
and volcanic lithic (DARL) 40Ar/39Ar dates (n = 2771)
from 30 modern river sediment samples document the spatial-temporal
evolution of Wrangell Arc magmatism, which includes construction of some of
the largest Quaternary volcanoes on Earth. Mismatches in DZ and DARL date
distributions highlight the impact of variables such as mineral fertility
and downstream mixing/dilution on resulting provenance signatures.
Geochronologic data document the initiation of Wrangell Arc magmatism at
ca. 30–17 Ma along both sides of the Totschunda fault on the north flank of
the Wrangell–St. Elias Mountains in Alaska, followed by southeastward
progression of magmatism at ca. 17–10 Ma along the Duke River fault in the
Yukon. This spatial-temporal evolution is attributable to dextral
translation along intra-arc, strike-slip faults and a change in the
geometry of the subducting slab (slab curling/steepening). Magmatism then
progressed generally westward outboard of the Totschunda and Duke River
faults at ca. 13–6 Ma along the southern flank of the Wrangell–St. Elias
Mountains in Alaska and then northwestward from ca. 6 Ma to present in the
western Wrangell Mountains. The 13 Ma to present spatial-temporal evolution
is consistent with dextral translation along intra-arc, strike-slip faults
and previously documented changes in plate boundary conditions, which
include an increase in plate convergence rate and angle at ca. 6 Ma.
Voluminous magmatism is attributed to shallow subduction-related flux
melting and slab edge melting that is driven by asthenospheric upwelling
along the lateral edge of the Yakutat flat slab. Magmatism was persistently
focused within or adjacent to a remnant suture zone, which indicates that
upper plate crustal heterogeneities influenced arc magmatism. Rivers
sampled also yield subordinate Paleozoic–Mesozoic DZ and DARL age
populations that reflect earlier episodes of magmatism within underlying
accreted terranes and match magmatic flare-ups documented along the
Cordilleran margin.
View article
:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02417.1/610425/Geochronology-of-the-Wrangell-Arc-Spatial-temporal
The spatial association of accessory minerals with biotite in granitic
rocks from the South Mountain Batholith, Nova Scotia, Canada
D. Barrie Clarke; Axel D. Renno; David C. Hamilton; Sabine Gilbricht; Kai
Bachmann
Abstract
:
We use mineral liberation analysis (MLA) to quantify the spatial
association of 15,118 grains of accessory apatite, monazite, xenotime, and
zircon with essential biotite, and clustered with themselves, in a
peraluminous biotite granodiorite from the South Mountain Batholith in Nova
Scotia (Canada). A random distribution of accessory minerals demands that
the proportion of accessory minerals in contact with biotite is identical
to the proportion of biotite in the rock, and the binary touching factor
(percentage of accessory mineral touching biotite divided by modal
proportion of biotite) would be ~1.00. Instead, the mean binary touching
factors for the four accessory minerals in relation to biotite are: apatite
(5.06 for 11,168 grains), monazite (4.68 for 857 grains), xenotime (4.36
for 217 grains), and zircon (5.05 for 2876 grains). Shared perimeter
factors give similar values. Accessory mineral grains that straddle biotite
grain boundaries are larger than completely locked, or completely
liberated, accessory grains. Only apatite-monazite clusters are
significantly more abundant than expected for random distribution. The
high, and statistically significant, binary touching factors and shared
perimeter factors suggest a strong physical or chemical control on their
spatial association. We evaluate random collisions in magma (synneusis),
heterogeneous nucleation processes, induced nucleation in passively
enriched boundary layers, and induced nucleation in actively enriched
boundary layers to explain the significant touching factors. All processes
operate during the crystallization history of the magma, but induced
nucleation in passively and actively enriched boundary layers are most
likely to explain the strong spatial association of phosphate accessories
and zircon with biotite. In addition, at least some of the apatite and
zircon may also enter the granitic magma as inclusions in grains of
Ostwald-ripened xenocrystic biotite.
View article
:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02339.1/610426/The-spatial-association-of-accessory-minerals-with
Cooling of the continental plate during flat-slab subduction
Xiaowen Liu; Claire A. Currie; Lara S. Wagner
Abstract
:
Most flat-slab subduction regions are marked by an absence of arc
volcanism, which is consistent with closure of the hot mantle wedge as the
subducting plate flattens below the continent. Farther inland, low surface
heat flow is observed, which is generally attributed to cooling of the
continent by the underlying flat slab. However, modern flat slabs have only
been in place for <20 Ma, and it is unclear whether there has been
sufficient time for cooling to occur. We use numerical models to assess
temporal variations in continental thermal structure during flat-slab
subduction. Our models show that the flat slab leads to continental cooling
on timescales of tens of millions of years. Cool slab temperatures must
diffuse through the continental lithosphere, resulting in a delay between
slab emplacement and surface cooling. Therefore, the timescales primarily
depend on the flat-slab depth with shallower slabs resulting in shorter
timescales. The magnitude of cooling increases for a shallow or long-lived
flat slab, old subducting plate, and fast convergence rates. For regions
with flat slabs at 45–70 km depth (e.g., Mexico and Peru), shallow
continental cooling initiates 5–10 Ma after slab emplacement, and low
surface heat flow in these regions is largely explained by the presence of
the flat slab. However, for the Pampean region in Chile, with an
~100-km-deep slab, our models predict that conductive cooling has not yet
affected the surface heat flow. The low heat flow observed requires
additional processes such as advective cooling from the infiltration of
fluids released through dehydration of the flat slab.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02402.1/610427/Cooling-of-the-continental-plate-during-flat-slab
Ultrahigh-temperature granulite-facies metamorphism and exhumation of
deep crust in a migmatite dome during late- to post- orogenic collapse
and extension in the central Adirondack Highlands (New York, USA)
Ellen P. Metzger; Mary L. Leech; Michael W. Davis; Jackson V. Reeder;
Brandon A. Swanson ...
Abstract:
This study combines field observations, mineral and whole-rock
geochemistry, phase equilibrium modeling, and U-Pb sensitive
high-resolution ion microprobe (SHRIMP) zircon geochronology to investigate
sillimanite-bearing felsic migmatites exposed on Ledge Mountain in the
central Adirondack Highlands (New York, USA), part of an extensive belt of
mid-crustal rocks comprising the hinterland of the Mesoproterozoic
Grenville orogen. Phase equilibrium modeling suggests minimum peak
metamorphic conditions of 960–1025 °C and 11–12.5 kbar during the Ottawan
orogeny—significantly higher pressure-temperature conditions than
previously determined—followed by a period of near-isothermal
decompression, then isobaric cooling. Petrography reveals abundant
melt-related microstructures, and pseudosection models show the presence of
at least ~15%–30% melt during buoyancy-driven exhumation and decompression.
New zircon data document late Ottawan (re)crystallization at ca. 1047 ± 5
to 1035 ± 2 Ma following ultrahigh-temperature (UHT) metamorphism and
anatexis on the retrograde cooling path. Inherited zircon cores give a mean
date of 1136 ± 5 Ma, which suggests derivation of these felsic granulites
by partial melting of older igneous rocks. The ferroan, anhydrous character
of the granulites is similar to that of the ca. 1050 Ma Lyon Mountain
Granite and consistent with origin in a late- to post-Ottawan extensional
environment. We present a model for development of a late Ottawan
migmatitic gneiss dome in the central Adirondacks that exhumed deep crustal
rocks including the Snowy Mountain and Oregon anorthosite massifs with UHT
Ledge Mountain migmatites. Recognition of deep crustal meta-plutonic rocks
recording UHT metamorphism in a migmatite gneiss dome has significant
implications for crustal behavior in this formerly thickened orogen.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02318.1/610079/Ultrahigh-temperature-granulite-facies
GEOSPHERE articles are available at
https://geosphere.geoscienceworld.org/content/early/recent
. Representatives of the media may obtain complimentary copies of GEOSPHERE
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