New GSA Bulletin Articles Published Ahead of Print in September
Boulder, Colo., USA: The Geological Society of America regularly publishes
articles online ahead of print. GSA Bulletin topics include the
nature and dynamics of China and Tibet; the Yellowstone hotspot; magma
mushes; the Barberton Greenstone Belt; juvenile hafnium; the nature and
origin of cratons constrained by their surface geology; and the push-pull
of Pelagonia. You can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Variscan intracrustal recycling by melting of Carboniferous arc-like
igneous protoliths (Évora Massif, Iberian Variscan belt)
Carmen Rodríguez; Manuel Francisco Pereira; Antonio Castro; Gabriel
Gutiérrez-Alonso; Carlos Fernández
Abstract:
Bulk rock geochemistry and sensitive high-resolution ion microprobe zircon
geochronology of igneous and metaigneous rocks of the Évora gneiss dome,
located to the north of the reworked Rheic Ocean suture zone in the
southwest Iberian Variscan belt, reveal a succession of magmatic and
melting events lasting ∼30 m.y. between ca. 341−314 Ma. The study of
detailed field relationships of orthomigmatites (i.e., migmatites from
igneous protoliths) and host granitic rocks proved to be crucial to
reconstruct the complex sequence of tectono-thermal events of the Évora
gneiss dome. The older igneous protoliths, with marked geochemical arc-like
signatures, are represented by 338 ± 3 Ma tonalites and 336 ± 3 Ma
diorites. These tonalites and diorites appear as mesosomes of igneous
orthomigmatites containing new melts (leucosomes) of monzogranite
composition and silica-poor trondhjemites formed in a melting episode at
329 ± 4/6 to 327 ± 3 Ma. The absence of peritectic phases (e.g., pyroxene),
together with shearing associated with migmatization, imply the existence
of water-rich fluids during melting of the older igneous rocks of the Évora
gneiss dome. This melting event is coeval with the second magmatic event of
the Évora gneiss dome represented by the neighboring Pavia pluton. A
porphyritic monzogranite dated at 314 ± 4 Ma defines a later magmatic
event. The porphyritic monzogranite encloses large blocks of the
orthomigmatites and contains magmatic mafic enclaves (autoliths) dated at
337 ± 4 Ma that are ∼23 m.y. older than the host rock. All studied rocks of
the Évora gneiss dome show arc-like, calc-alkaline geochemical signatures.
Our results support recycling of intermediate-mafic plutonic rocks,
representing the root of an early magmatic arc that formed at the time of
Gondwana-Laurussia convergence (after the closure of the Rheic Ocean) and
coeval subduction of the Paleotethys. A geodynamic model involving ridge
subduction is proposed to explain the Early Carboniferous intra-orogenic
crustal extension, dome formation, exhumation of high-grade rocks,
compositional variations of magmatism and formation of new granitic
magmatism in which, arc-like signatures were inherited from the crustal
source.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36111.1/607910/Variscan-intracrustal-recycling-by-melting-of
Fluid inclusion evidence for the magmatic-hydrothermal evolution of
closely linked porphyry Au, porphyry Mo, and barren systems, East
Qinling, China
Li Tang; Thomas Wagner; Tobias Fusswinkel; Shou-Ting Zhang; Xin-Kai Hu ...
Abstract:
The Xiong’ershan district in central China hosts broadly coeval porphyry Au
(Qiyugou deposit), porphyry Mo (Leimengou deposit), and barren (Huashan
pluton) systems. The key controls on the ore potential and different
mineralization styles in these systems are not well understood, with
first-order differences in fluid chemistry and melt sources being the main
alternatives. The fluid inclusion characteristics of all three porphyry
systems have been studied using an integrated approach that combines field
geology, petrography, microthermometry, and laser ablation−inductively
coupled plasma−mass spectrometry analysis of single fluid inclusions. The
results permit a reconstruction of the magmatic-hydrothermal evolution of
the ore-forming fluids, and to elucidate whether specialized hydrothermal
fluids strongly enriched in ore metals (i.e., Mo, Au, Cu) were essential to
form the economically significant deposits. The fluid compositions across
the three hydrothermal stages from the Qiyugou Au deposit remain
approximately the same over time, suggesting that progressive magma
fractionation, fluid-rock reaction along fluid path, and mineral
precipitation had a limited effect on fluid composition. The syn-ore stage
fluids of the Leimengou Mo deposit are characterized by higher Cs/Na,
Sr/Na, and B/Na, but lower K/Na and Cl/Na ratios, and also have salinities
and homogenization temperatures distinct from the earlier fluids. This
demonstrates that Mo mineralization was caused by a second pulse of fluid
input from a highly fractionated felsic magma subsequent to the pre-ore
stage. At the Huashan barren pluton, fluids from phase II have higher
Cs/Na, B/Na, Li/Na, and Rb/Na ratios with lower homogenization temperatures
than fluids occurring in porphyritic rocks of phase III, reflecting a
higher degree of magma fractionation of this plutonic complex. The Huashan
pluton does not host economic mineralization which is likely caused by the
low ore metal tenor, inefficient fluid extraction from the melt, or the
flat-roof geometry preventing accumulation of a large volume of fluid in
the apical part. The Au tenor of the Qiyugou deposit was most likely
contributed by mantle-derived material of higher Mg/Na, Fe/Na, Pb/Na, and
Zn/Na ratios. Taken together, the metal charged magmatic-hydrothermal
fluids, steeply dipping geometry, and small volume of the porphyry stocks
all suggest that a much larger magma chamber feeding the porphyry systems
should be present at deeper levels with good potential for Mo
mineralization below the current level of exposure at Qiyugou deposit.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36170.1/607861/Fluid-inclusion-evidence-for-the-magmatic
Paleogeographic position of the central Dodecanese Islands,
southeastern Greece: The push-pull of Pelagonia
B. Grasemann; D.A. Schneider; K. Soukis; V. Roche; B. Hubmann
Abstract:
The paleogeographic position of the central Dodecanese Islands at the
transition between the Aegean and Anatolian plates plays a considerable
role in understanding the link between both geologically unique domains. In
this study, we investigate the tectonic history of the central Dodecanese
Islands and the general correlation with the Aegean and western Anatolian
and focus on the poorly studied islands of Kalymnos and Telendos. Three
different major tectonic units were mapped on both islands from bottom to
top: (1) The Kefala Unit consists of late Paleozoic, fossil-rich
limestones, which have been deformed into a SE-vergent fold-and-thrust belt
sealed by an up to 200-m-thick wildflysch-type olistostrome with marble and
ultramafic blocks on a scale of tens of meters. (2) The Marina Basement
Unit consists of a Variscan amphibolite facies basement with garnet mica
schists, quartzites, and amphibolites. (3) Verrucano-type formation violet
shales and Mesozoic unmetamorphosed limestones form the Marina Cover Unit.
Correlation of these units with other units in the Aegean suggests that
Kalymnos is paleogeographically located at the southern margin of the
Pelagonian domain, and therefore it was in a structurally upper tectonic
position during the Paleogene Alpine orogeny. New white mica 40
Ar/39Ar ages confirm the Carboniferous deformation of the Marina
Basement Unit followed by a weak Triassic thermal event. Single-grain white
mica 40Ar/39Ar ages from pressure solution cleavage
of the newly defined Telendos Thrust suggest that the Marina Basement Unit
was thrusted toward the north on top of the Kefala Unit in the Paleocene.
Located at a tectonically upper position, the units exposed in the central
Dodecanese escaped subduction and the syn-orogenic, high-pressure
metamorphism. However, these units were affected by post-orogenic
extension, and the contact between the Marina Basement Unit and the
non-metamorphic Marina Cover Unit has been reactivated by the cataclastic
top-to-SSW, low-angle Kalymnos Detachment. Zircon (U-Th)/He ages from the
Kefala and Marina Basement Units are ca. 30 Ma, which indicates that
exhumation and cooling below the Kalymnos Detachment started in the
Oligocene. Conjugate brittle high-angle normal fault systems, which
resulted in the formation of four major WNW-ESE−trending graben systems on
Kalymnos, localized mainly in the Marina Cover Unit and probably rooted in
the mechanically linked Kalymnos Detachment. Since Oligo-Miocene
deformation in the northern Dodecanese records top-to-NNE extension and the
Kalymnos Detachment accommodated top-to-SSW extension, we suggest that
back-arc extension in the whole Aegean realm and transition to the
Anatolian plate is bivergent.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36095.1/607820/Paleogeographic-position-of-the-central-Dodecanese
Stratigraphy around the Cretaceous-Paleogene boundary in sediment cores
from the Lord Howe Rise, Southwest Pacific
Junichiro Kuroda; Kyoko Hagino; Yoichi Usui; Paul R. Bown; Kan-Hsi Hsiung
...
Abstract:
During Deep Sea Drilling Project (DSDP) Leg 21, Cenozoic and latest
Cretaceous sediments were recovered at Site 208 on the Lord Howe Rise,
Southwest Pacific. We provide new biostratigraphic, magnetostratigraphic
and chemostratigraphic data from Site 208 to constrain the stratigraphy
around the Cretaceous-Paleogene (K-Pg) boundary and to determine the depth
of the K-Pg boundary more precisely. Biostratigraphic data from calcareous
nannofossils indicate a near-continuous succession of sediments from the
mid-Maastrichtian (Late Cretaceous) to lowermost Thanetian (Paleocene) at
depths of 540−590 m below seafloor (mbsf). The biostratigraphic data
suggest that the K-Pg boundary corresponds to a siliceous claystone at the
base of an interval of silicified sediments (576.0−576.8 mbsf). Carbonate
carbon isotopic composition (δ13Ccarb) reveals a
negative shift across this interval, which is consistent with global
patterns of δ13C across the K-Pg boundary. Osmium concentration
and Os isotopic composition (187Os/188Os) can also be
used to identify the K-Pg boundary interval, as it is marked by a peak in
Os concentration and a drop in 187Os/188Os values to
0.12−0.15, both of which are the result of the Chicxulub impact event. Our 187Os/188Os data show trends similar to those of
coeval global seawater with the lowest value of 0.12−0.16 in the siliceous
claystone (576.8 mbsf). However, the concentration of Os is low (<80 pg
g−1) in this sample, which suggests that this siliceous
claystone was deposited around the K-Pg boundary but may not include the
boundary itself. Although the sedimentary record across the K-Pg interval
at Site 208 may not be completely continuous, it nevertheless captures a
time interval that is close to the Chicxulub impact event.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36112.1/607821/Stratigraphy-around-the-Cretaceous-Paleogene
Metallogenic fingerprint of a metasomatized lithospheric mantle feeding
gold endowment in the western Mediterranean basin
Erwin Schettino; Claudio Marchesi; José María González-Jiménez; Edward
Saunders; Károly Hidas ...
Abstract:
Spinel peridotite xenoliths (one plagioclase-bearing) hosted in alkaline
basalts from Tallante (southeast Spain) record the mineralogical and
geochemical fingerprint of the subcontinental lithospheric mantle (SCLM)
evolution beneath the southern Iberian margin. Mantle metasomatism in
fertile lherzolites caused the crystallization of clinopyroxene +
orthopyroxene + spinel clusters through the percolation of Miocene
subalkaline melts during the westward migration of the subduction front in
the western Mediterranean. In the Pliocene, heat and volatiles provided by
alkaline host-magmas triggered very low melting degrees of metasomatic
pyroxene-spinel assemblages, producing melt quenched to silicate glass and
reactive spongy coronae around clinopyroxene and spinel. Refertilization of
the Tallante peridotites induced the precipitation of base-metal sulfides
(BMS) included in metasomatic clino- and orthopyroxene. These sulfides
consist of pentlandite ± chalcopyrite ± bornite aggregates with homogeneous
composition in terms of major elements (Ni, Fe, Cu) and semi-metals (Se,
As, Te, Sb, Bi), but with wide variability of platinum-group elements (PGE)
fractionation (0.14 < PdN/IrN < 30.74).
Heterogeneous PGE signatures, as well as the presence of euhedral
Pt-Pd-Sn-rich platinum-group minerals (PGM) and/or Au-particles within BMS,
cannot be explained by conventional models of chalcophile partitioning from
sulfide melt. Alternatively, we suggest that they reflect the incorporation
of distinct populations of BMS, PGM, and metal nanoparticles (especially of
Pt, Pd, and Au) during mantle melting and/or melt percolation. Therefore,
we conclude that Miocene subalkaline melts released by asthenosphere
upwelling upon slab tearing of the Iberian continental margin effectively
stored metals in metasomatized domains of this sector of the SCLM.
Remarkably high Au concentrations in Tallante BMS (median 1.78 ppm) support
that these metasomatized domains provided a fertile source of metals,
especially gold, for the ore-productive Miocene magmatism of the
westernmost Mediterranean.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36065.1/607788/Metallogenic-fingerprint-of-a-metasomatized
The nature and origin of cratons constrained by their surface geology
A.M. Celal Şengör; Nalan Lom; Ali Polat
Abstract:
To the memory of Nicholas John (Nick) Archibald (1951−2014), master of
cratonic geology. Cratons, defined by their resistance to deformation, are
guardians of crustal and lithospheric material over billion-year time
scales. Archean and Proterozoic rocks can be found in many places on earth,
but not all of them represent cratonic areas. Some of these old terrains,
inappropriately termed “cratons” by some, have been parts of mobile belts
and have experienced widespread deformations in response to
mantle-plume-generated thermal weakening, uplift and consequent extension
and/or various plate boundary deformations well into the Phanerozoic. It is
a common misconception that cratons consist only of metamorphosed
crystalline rocks at their surface, as shown by the indiscriminate
designation of them by many as “shields.” Our compilation shows that this
conviction is not completely true. Some recent models argue that craton
formation results from crustal thickening caused by shortening and
subsequent removal of the upper crust by erosion. This process would expose
a high-grade metamorphic crust at the surface, but greenschist-grade
metamorphic rocks and even unmetamorphosed supracrustal sedimentary rocks
are widespread on some cratonic surfaces today, showing that craton
formation does not require total removal of the upper crust. Instead, the
granulitization of the roots of arcs may have been responsible for weighing
down the collided and thickened pieces and keeping their top surfaces
usually near sea level. In this study, we review the nature and origin of
cratons on four well-studied examples. The Superior Province (the Canadian
Shield), the Barberton Mountain (Kaapvaal province, South Africa), and the
Yilgarn province (Western Australia) show the diversity of rocks with
different origin and metamorphic degree at their surface. These fairly
extensive examples are chosen because they are typical. It would have been
impractical to review the entire extant cratonic surfaces on earth today.
We chose the inappropriately named North China “Craton” to discuss the
requirements to be classified as a craton.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36079.1/607789/The-nature-and-origin-of-cratons-constrained-by
Crustal deformation and exhumation within the India-Eurasia oblique
convergence zone: New insights from the Ailao Shan-Red River shear zone
B. Zhang; S.Y. Chen; Y. Wang; P.W. Reiners; F.L. Cai ...
Abstract:
During the collision of India and Eurasia, regional-scale strike-slip shear
zones played a key role in accommodating lateral extrusion of blocks, block
rotation, and vertical exhumation of metamorphic rocks as presented by
deformation on the Ailao Shan-Red River shear zone (ARSZ) in the Eastern
Himalayan Syntaxis region and western Yunnan, China. We report structural,
mica Ar/Ar, apatite fission-track (AFT), and apatite (U-Th)/He (AHe) data
from the Diancangshan massif in the middle segment of the ARSZ. These
structural data reveal that the massif forms a region-scale antiform,
bordered by two branches of the ARSZ along its eastern and western margins.
Structural evidence for partial melting in the horizontal mylonites in the
gneiss core document that the gneiss experienced a horizontal shear
deformation in the middle crust. Muscovite Ar/Ar ages of 36−29 Ma from the
core represent cooling ages. Muscovite Ar/Ar ages of 25 and 17 Ma from
greenschist-facies mylonites along the western and southern shear zones,
respectively, are interpreted as recording deformation in the ARSZ. The AFT
ages, ranging from 15 to 5 Ma, represent a quiescent gap with a slow
cooling/exhumation in the massif. AHe results suggest that a rapid cooling
and final exhumation episode of the massif could have started before 3.2
Ma, or likely ca. 5 Ma, and continue to the present. The high-temperature
horizontal shearing layers of the core were first formed across the
Indochina Block, locally antiformed along the tectonic boundaries, and then
cooled through the mica Ar-Ar closure temperature during Eocene or early
Oligocene, subsequently reworked and further exhumed by sinistral
strike-slip movement along the ARSZ during the early Oligocene (ca. 29 Ma),
lasting until ca. 17 Ma, then final exhumation of the massif occurred by
dextral normal faulting on the Weixi-Qiaohou and Red River faults along the
limbs of the ARSZ since ca. 5 Ma. The formation of the antiform could
indicate local crustal thickening in an early transpressional setting
corresponding to India-Asia convergence. Large-scale sinistral ductile
shear along the ARSZ in the shallow crust accommodated lateral extrusion of
the Indochina Block, and further contributed to the vertical exhumation of
the metamorphic massif from the late Oligocene to the middle Miocene.
Furthermore, the change of kinematic reversal and associated cooling
episodes along the ARSZ since the middle Miocene or early Pliocene imply a
tectonic transfer from strain localization along the major tectonic
boundaries to continuous deformation corresponding to plateau growth and
expansion.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35975.1/607790/Crustal-deformation-and-exhumation-within-the
Carbon and nitrogen isotope evidence for widespread presence of anoxic
intermediate waters before and during the Permian-Triassic mass
extinction
Baojin Wu; Genming Luo; Michael M. Joachimski; Paul B. Wignall; Lidan Lei
...
Abstract:
The largest mass extinction since the advent of animals occurred during the
Permian-Triassic (P-Tr) transition, ca. 252 Ma, and is commonly attributed
to the eruption of the Siberian Traps large igneous province. However, the
direct killing mechanism is still debated. In this study, we investigated
marine redox conditions of the intermediate water column that most
organisms inhabit with special attention to the time interval before the
onset of the mass extinction. The carbon isotope composition of carbonate
and organic carbon (δ13Ccarb and δ13C org) as well as the nitrogen isotope composition of bulk
nitrogen (δ15N) were analyzed in four P-Tr boundary sequences
(Zhongli, Jianshi, Ganxi, and Chaotian sections), which record a transect
from a shallow water carbonate platform to a deep water, lower ramp slope
in South China. δ13Ccarb shows a distinct negative
shift in all sections and displays a clear, 2−4‰, decreasing gradient
accompanying an increase in water depth both before and after the mass
extinction. A distinct negative shift in δ15N is observed in the
shallow water Zhongli section, whereas a minor negative shift is present in
the three deeper water sections. Before the mass extinction, the δ 15N values from shallow water sections are higher than those
from deeper waters. The low δ15N values close to 0‰ in deeper
water sections suggest that microbial nitrogen fixation was the predominant
source of biologically available nitrogen before the onset of the mass
extinction. Thus, the water depth- dependent gradient in δ13C carb and δ15N suggests that an oxygen-deficient
intermediate water column was already present before the mass extinction.
The uniform δ15N values around 0‰ accompanying the onset of the
mass extinction reveal that anoxic intermediate waters expanded into
shallow waters. Meanwhile, the distinct positive shift in δ13C org observed in upper ramp slope sections, i.e., the Jianshi and
Ganxi sections, suggests that a euxinic photic zone was at least
episodically present in the earliest Triassic. The temporal coincidence
between the expansion of intermediate water column anoxia and the onset of
the P-Tr mass extinction supports the hypothesis that marine anoxia was a
direct killing mechanism.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36005.1/607747/Carbon-and-nitrogen-isotope-evidence-for
The Middle-Late Cretaceous Zagros ophiolites, Iran: Linking of a 3000
km swath of subduction initiation fore-arc lithosphere from Troodos,
Cyprus to Oman
H.S. Moghadam; Q.L. Li; W.L. Griffin; M. Chiaradia; K. Hoernle ...
Abstract:
New trace-element, radiogenic Sr-Nd-Pb isotopic and geochronological data
from Middle-Late Cretaceous Zagros ophiolites of Iran give new insights
into the tectono-magmatic history of these supra-subduction zone (SSZ)-type
ophiolites. The distribution of Middle-Late Cretaceous SSZ-type ophiolites
in Iran comprises two parallel belts: (1) the outer Zagros ophiolitic belt
and (2) the inner Zagros ophiolitic belt. These Middle-Late Cretaceous
ophiolites were generated by seafloor spreading in what became the fore-arc
and back-arc during the subduction initiation event and now define a
∼3000-km-long belt from Cyprus to Turkey, Syria, Iran, the UAE, and Oman.
The Zagros ophiolites contain complete (if disrupted) mantle and crustal
sequences. Mantle sequences from both outer-belt and inner-belt ophiolites
are dominated by dunites, harzburgites, and lherzolites with minor
chromitite lenses. Peridotites are also intruded by gabbros and a variety
of mafic to minor felsic (plagiogranite and dacite) dikes. Crustal rocks
comprise ultramafic-mafic cumulates as well as isotropic gabbros, sheeted
dike complexes, pillowed and massive lavas, and felsic rocks. Our new
zircon U-Pb ages indicate that the outer-belt and inner-belt ophiolites
formed near coevally during the Middle-Late Cretaceous; 100−96 Ma for the
outer belt and 105−94 Ma for the inner belt. Both incompatible-element
ratios and isotopic data confirm that depleted mantle and variable
contributions of subduction components were involved in the genesis of
outer-belt and inner-belt rocks. Our data for the outer belt and inner belt
along with those from better-studied ophiolites in Cyprus, Turkey, the UAE,
and Oman lead to the conclusion that a broad, ∼3000-km-long swath of
fore-arc lithosphere was created during Middle-Late Cretaceous time.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36041.1/607748/The-Middle-Late-Cretaceous-Zagros-ophiolites-Iran
Juvenile hafnium isotopic compositions recording a late
Carboniferous−Early Triassic retreating subduction in the southern
Central Asian Orogenic Belt: A case study from the southern Alxa
Rongguo Zheng; Jinyi Li; Jin Zhang
Abstract:
Two successive and parallel magmatic arcs within the southern Alxa provide
an ideal area to examine the influence of tectonic switching on temporal
and spatial distribution of magmatism within accretionary orogens. This
study presents new geochronological and geochemical data for Yingen and
Quagu plutons from the southern Alxa, located in the southern Central Asian
Orogenic Belt. Late Permian Yingen granitic dikes (ca. 252 Ma) have
depleted whole-rock Nd isotopic compositions, high Sr, low Y and Yb, and
high Sr/Y ratios, all of which indicate they were generated by the partial
melting of subducted young/hot oceanic slab. The Middle Permian (271 Ma)
Yingen hosting granites contain elevated contents of Nb and Zr, and have
high 10,000 × Ga/Al ratios, suggesting that they resulted from mixing
between Neoproterozoic crust-derived felsic magmas and depleted
mantle-derived mafic magmas. The Quagu pluton yields ca. 271−262 Ma zircon
U-Pb ages and has an adakitic high-Mg diorite-like geochemical composition,
suggesting that it originated from interaction between slab-derived melts
and overlying peridotite material. Collectively, these data record the
subduction of the Enger Us oceanic slab beneath
Mesoproterozoic−Neoproterozoic sialic crust, generating a Japan-type arc
within the southern Alxa during Middle−Late Permian. Temporal-spatial
variations of zircon Hf isotope for plutons suggest tectonic switching from
advancing to retreating subduction during Carboniferous−Early Triassic
within the southern Alxa. An advancing subduction resulted from the
subduction of the Paleo-Asian Ocean, and a retreating subduction was
related to plate boundary reorganization during the assembly of Pangea.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35991.1/607653/Juvenile-hafnium-isotopic-compositions-recording-a
Coupled magmatic and host rock processes during the initiation of the
Tuolumne Intrusive Complex, Sierra Nevada, California, USA: A
transition from ephemeral sheets to long-lived, active magma mushes
Valbone Memeti; Scott R. Paterson; Roland Mundil
Abstract:
The initiation of pluton formation is rarely preserved as the rock record
is typically overprinted by younger intruding pulses. An exception is the
80 km2 Kuna Crest lobe, which marks the initiation of the 95−85
Ma, 1100 km2 Tuolumne Intrusive Complex in the Sierra Nevada,
California, USA. We present a detailed map of the lithologies and structure
of the Kuna Crest lobe, associated sheeted complex and satellite plutons,
and their host rocks, using chemical abrasion−isotope dilution−thermal
ionization mass spectrometry and laser ablation−inductively coupled
plasma−mass spectrometry U-Pb zircon geochronology, element and isotope
geochemistry, and Al-in-hornblende thermobarometry to conclude the
following: (a) The 94.91 ± 0.53 Ma to 92.75 ± 0.11 Ma Kuna Crest lobe and
its marginal sheeted complex preserved the oldest intrusive pulses and most
mantle-like compositions of the entire Tuolumne Intrusive Complex. (b)
Emplacement began with magma wedging of low volume magma pulses resulting
in a sheeted complex that is compositionally heterogeneous at outcrop
scales, but isotopically homogeneous. (c) These early magmas established a
pre-heated pathway within just a few hundreds of thousands of years that
gave way to the formation of the ∼1.5 million-year-long active,
compositionally more homogeneous but isotopically more heterogeneous magma
mush across the Kuna Crest lobe. The host rocks and previously intruded
magma were displaced largely vertically through downward flow. (d) The
melt-interconnected mush zone in the lobe allowed for magma mixing and
crystal-liquid separation at the emplacement level. We interpret this lobe
to represent an upper- to mid-crustal, vertical magma transfer zone that
likely fed shallower plutons and potentially volcanic eruptions. We propose
a filter pressing mechanism driven by vertical magma transport through the
lobe resulting in margin-parallel fabrics, plagioclase-rich crystal
cumulates, inward draining and upward loss (of up to 40%) of interstitial
melts. Some inward drained melts hybridized with later intruding Half Dome
magmas at the transition to the main Tuolumne Intrusive Complex. Some of
the lobe magmas, including fractionated melts, drained laterally into the
strain shadow of the lobe to form the satellite plutons, further
contributing to cumulate formation in the lobe. This study documents that
within only a few hundreds of thousands of years, arc magma plumbing
systems are capable of establishing a focused magma pathway to build up to
increasingly larger magma bodies that are capable of undergoing magma
differentiation and feeding shallower plutons and volcanic eruptions.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35871.1/607654/Coupled-magmatic-and-host-rock-processes-during
Progressive accretion recorded in sedimentary rocks of the 3.28−3.23 Ga
Fig Tree Group, Barberton Greenstone Belt
Nadja Drabon; Donald R. Lowe
Abstract:
One of the major challenges in early Earth geology is the interpretation of
the nature of the crust and tectonic processes due to the limited exposures
of Archean rocks. This question is predominantly addressed by numerical
modeling, structural geology, geochemical analyses, and petrological
approaches. Here we report on the reconstruction of one of the oldest,
well-preserved volcano-sedimentary sequences on Earth, the 3.28−3.22 Ga Fig
Tree Group in the Barberton Greenstone Belt, South Africa, based on
geochronology, provenance, and stratigraphy to provide new constraints on
the nature of tectonic processes in the Archean. The Fig Tree basin was
asymmetric and the onset of deposition varied across the greenstone belt.
The Fig Tree Group is now preserved in east-west oriented bands of
fault-bounded structural belts with those preserved in the southern parts
of the greenstone belt showing an onset of deposition at 3.28 Ga, those in
the center at 3.26 Ga, and those in the north at 3.24 Ga.
Stratigraphically, the rocks display a general up-section trend from deeper
to shallower-water deposition and/or from finer- to coarser-grained
sedimentary rocks. Associated with this up-section stratigraphic trend, the
sedimentary rocks show a change in provenance from more regionally similar
to more locally variable, and an increase in felsic volcanic activity,
especially toward the closure of Fig Tree deposition. The data is
consistent with formation of the Fig Tree Group in a compressional tectonic
setting by deposition in a foreland basin that experienced progressive
accretion of crustal terranes onto a northward prograding fold-and-thrust
belt.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35973.1/607507/Progressive-accretion-recorded-in-sedimentary
From subduction initiation to hot subduction: Life of a Neoarchean
subduction zone from the Dengfeng Greenstone Belt, North China Craton
Hao Deng; Ning Jia; Timothy Kusky; Ali Polat; Guanglei Peng ...
Abstract:
We report a spatially and temporally linked arc magmatic association of
2.55−2.50 Ga mid-ocean ridge basalt (MORB)-affinity gabbros, arc-affinity
basalts, high-Mg basalts/basaltic andesites (HMBA), Nb-enriched basalts
(NEB), and a tonalite-trondhjemite-granodiorite (TTG)-like granodiorite
from the Dengfeng greenstone belt (DFGB), North China Craton. These are
identical to arc magmatic rock suites from hot subduction in the present
plate mosaic, and resemble suites formed during subduction initiation.
Laser ablation−inductively coupled plasma−mass spectrometry zircon U-Pb
dating indicates that gabbros and granodiorites intruded at ca. 2.55 Ga and
the HMBAs formed at ca. 2.50 Ga, indicating a long duration of subduction
zone magmatic activity of at least 50 m.y. in the DFGB. The gabbros are
characterized by flat light-rare earth element (LREE) patterns without
negative Nb and Zr anomalies, showing a MORB-like geochemical affinity.
Combined with previously reported normal-MORB-type basalts, we suggest that
they may represent magmatic products during intraoceanic subduction
initiation where their mantle source was slightly metasomatized by
subduction-derived fluids during the initiation stage. The arc-affinity
basalts are characterized by slightly enriched LREE patterns with negative
Nb and Zr anomalies. The HMBAs have higher contents of MgO, Ni, and Cr and
display more fractionated rare earth element (REE) patterns and large
negative Nb and Zr anomalies than the arc-affinity basalts. The NEBs are
characterized by high absolute contents of Nb (9.9−14 ppm) and high ratios
of Nb/Laprimitive mantle (pm) (0.40−0.46) and Nb/Thpm
(0.44−0.54), and moderately fractionated REE and minor negative Nb
anomalies with no Zr anomalies. One granodiorite sample is characterized by
a highly fractionated REE pattern and negative Nb and Zr anomalies.
Geochemical modeling suggests that the association of the normal
arc-affinity basalt (NAB)-HMBA-NEB can be interpreted to be generated by
hybridization of mantle wedge peridotites by slab-derived TTG-like melts.
Positive values of whole-rock εNd (t) and zircon εHf (t) of the 2.55−2.50
Ga magmatic associations are consistent with a long-term depleted mantle
source for the mafic assemblage. The 2.55−2.50 Ga magmatic association from
MORB-affinity gabbros and basalts to arc-affinity NAB-HMBA-NEB may record a
continuous Neoarchean geodynamic process from intraoceanic subduction
initiation to mature arc magmatism. The NAB-HMBA-NEB association with
regional sanukitoids and high-Al TTGs indicates that hot subduction may
have played an important role in the production of arc-related magmatism
during the Archean.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35994.1/607508/From-subduction-initiation-to-hot-subduction-Life
Building a continental arc section: Constraints from Paleozoic
granulite-facies metamorphism, anatexis, and magmatism in the northern
margin of the Qilian Block, northern Tibet Plateau
Yinbiao Peng; Shengyao Yu; Jianxin Zhang; Yunshuai Li; Sanzhong Li ...
Abstract:
Continental arcs in active continental margins (especially deep-seated arc
magmatism, anatexis, and metamorphism) can be extremely significant in
evaluating continent building processes. In this contribution, a Paleozoic
continental arc section is constructed based on coeval granulite-facies
metamorphism, anatexis, and magmatism on the northern margin of the Qilian
Block, which record two significant episodes of continental crust growth.
The deeper layer of the lower crust mainly consists of medium-high pressure
mafic and felsic granulites, with apparent peak pressure-temperature
conditions of 11−13 kbar and 800−950 °C, corresponding to crustal depths of
∼35−45 km. The high-pressure mafic granulite and local garnet-cumulate
represent mafic residues via dehydration melting involving breakdown of
amphibole with anatectic garnet growth. Zircon U-Pb geochronology indicates
that these high-grade metamorphic rocks experienced peak granulite-facies
metamorphism at ca. 450 Ma. In the upper layer of the lower crust, the most
abundant rocks are preexisting garnet-bearing metasedimentary rocks,
orthogneiss, and local garnet amphibolite, which experienced
medium-pressure amphibolite-facies to granulite-facies metamorphism at
depths of 20−30 km at ca. 450 Ma. These metasedimentary rocks and
orthogneiss have also experienced partial melting involving mica and rare
amphibole at 457−453 Ma. The shallow to mid-crust is primarily composed of
diorite-granodiorite batholiths and volcanic cover with multiple origin,
which were intruded during 500−450 Ma, recording long-term crustal growth
and differentiation episode. As a whole, two episodes of continental crust
growth were depicted in the continental arc section on the northern margin
of the Qilian Block, including: (a) the first episode is documented in a
lithological assemblage composing of coeval mafic-intermediate intrusive
and volcanic rocks derived from partial melting of modified lithospheric
mantle and subducted oceanic crust during southward subduction of the North
Qilian Ocean at 500−480 Ma; (b) the second episode is recorded in mafic
rocks derived from partial melting of modified lithospheric mantle during
transition from oceanic subduction to initial collision at 460−450 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36100.1/607509/Building-a-continental-arc-section-Constraints
Clay templates in Ediacaran vendotaeniaceans: Implications for the
taphonomy of carbonaceous fossils
B. Becker-Kerber; A. Abd Elmola; A. Zhuravlev; C. Gaucher; M.G. Simões ...
Abstract:
Although rare, sedimentary deposits containing exceptionally preserved
fossils (i.e., Lagerstätten) have shaped our view on the history of life at
particular intervals, such as those recording the Cambrian radiation of
animals. Therefore, understanding the processes that lead to the
fossilization of unmineralized tissues is crucial to better interpret these
fossil assemblages. A key issue on the fossilization of exceptionally
preserved fossils is linked to the role of clay minerals in the
high-fidelity preservation of recalcitrant and soft tissues. Here, we show
for the first time, an association of unusual fibrous clays with
carbonaceous fossils (Vendotaenia) in the late Ediacaran Tamengo
Formation (Mato Grosso do Sul State, western Brazil). The vendotaeniaceans
occur in laminated mudstones/siltstones interpreted as being deposited in
outer to distal mid-ramp depositionary settings. The fossils are
characterized by ribbon-shaped compressions 0.56 mm in mean width. The
fibrous clays are obliquely oriented with respect to the bedding plane, and
follow the orientation of tectonically deformed structures. Our
mineralogical, geochemical, and petrographic data demonstrate that these
clays are mainly composed of chlorite-smectite mixed layered minerals, with
>50% chlorite. Altogether, our results suggest that these fibrous
minerals formed in the late-diagenetic zone to lower anchizone, reinforcing
the previous idea that clay minerals associated with fossils are not
necessarily related to the preservation of soft tissues. Instead, the
initial preservative pathway in our fossils was probably restricted to
organic matter conservation in reducing fine-grained sediments, similar to
other deposits with carbonaceous fossils. This newly established mechanism,
which involves the formation of clays on organic templates in the
late-diagenetic zone, is likely a more widespread phenomenon than
previously thought.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36033.1/607510/Clay-templates-in-Ediacaran-vendotaeniaceans
Patterns of incision and deformation on the southern flank of the
Yellowstone hotspot from terraces and topography
Daphnee Tuzlak; Joel Pederson; Aaron Bufe; Tammy Rittenour
Abstract:
Understanding the dynamics of the greater Yellowstone region requires
constraints on deformation spanning million year to decadal timescales, but
intermediate-scale (Quaternary) records of erosion and deformation are
lacking. The Upper Snake River drainage crosses from the uplifting region
that encompasses the Yellowstone Plateau into the subsiding Snake River
Plain and provides an opportunity to investigate a transect across the
trailing margin of the hotspot. Here, we present a new chronostratigraphy
of fluvial terraces along the lower Hoback and Upper Snake Rivers and
measure drainage characteristics through Alpine Canyon interpreted in the
context of bedrock erodibility. We attempt to evaluate whether incision is
driven by uplift of the Yellowstone system, subsidence of the Snake River
Plain, or individual faults along the river’s path. The Upper Snake River
in our study area is incising at roughly 0.3 m/k.y. (300 m/m.y.), which is
similar to estimates from drainages at the leading eastern margin of the
Yellowstone system. The pattern of terrace incision, however, is not
consistent with widely hypothesized headwater uplift from the hotspot but
instead is consistent with downstream baselevel fall as well as localized
deformation along normal faults. Both the Astoria and Hoback faults are
documented as active in the late Quaternary, and an offset terrace
indicates a slip rate of 0.25−0.5 m/k.y. (250−500 m/m.y.) for the Hoback
fault. Although tributary channel steepness corresponds with bedrock
strength, patterns of χ across divides support baselevel fall to
the west. Subsidence of the Snake River Plain may be a source of this
baselevel fall, but we suggest that the closer Grand Valley fault system
could be more active than previously thought.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35923.1/607511/Patterns-of-incision-and-deformation-on-the
Three-stage modification of lithospheric mantle: Evidence from
petrology, in-situ trace elements, and Sr isotopes of mantle xenoliths
in the Cenozoic basalts, northeastern North China Craton
Xinran Xu; Yanjie Tang; Jifeng Ying; Xinmiao Zhao; Yan Xiao
Abstract:
We present mineralogical and geochemical compositions of mantle xenoliths
from two Cenozoic basalt localities of the northeastern North China Craton.
These xenoliths include lherzolite, harzburgite, and websterite. They are
generally fertile in major elements and different from the typical cratonic
lithosphere, which is consistent with previous hypotheses regarding craton
destruction. The ratios of 87Sr/86Sr and (La/Yb) N of clinopyroxenes (Cpx) in one lherzolite are relatively low
in the core but high in the rim. The center of the Cpx grain has a high U
concentration. Changes in trace elements and Sr isotopes indicate that
later stage high 87Sr/86Sr melt metasomatism
superimposed on the early hydrous melt/fluid. The Cpxs in some xenoliths
are low in Ti/Eu but high in Ca/Al and light rare earth elements, which
indicates carbonate melt metasomatism. 87Sr/86Sr is
increased in the core and decreased in the rim of most Cpx grains, which
reflects the superposition of two-stage metasomatism. The early agent
should be high in 87Sr/86Sr, and the recent agent
should be low in 87Sr/86Sr. The Cpxs in olivine
websterite are low in 87Sr/86Sr (0.70220−0.70320),
which reflects the recent metasomatism of asthenosphere-derived melt.
Collectively, these observations reflect a three-stage modification of the
lithospheric mantle. First-stage hydrous melt/fluid could come from the
dehydration of young subducted plates. Second-stage melt/fluid of high 87Sr/86Sr could derive from the partial melting of
the subducted altered oceanic crust, and the recent melt/fluid of low 87Sr/86Sr should be from the asthenosphere.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36140.1/607462/Three-stage-modification-of-lithospheric-mantle
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