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New GSA Bulletin articles published ahead of print in April

Boulder, Colo., USA: The Geological Society of America regularly publishes
articles online ahead of print. For April, GSA Bulletin topics
include multiple articles about the dynamics of China and Tibet; the Bell
River hypothesis that proposes that an ancestral, transcontinental river
occupied much of northern North America during the Cenozoic Era; new
findings in the climatic history during one of the Earth’s coldest periods:
The Late Paleozoic Ice Age; and the age an nature of the Chicxulub impact
crater. You can find these articles at


Evidence of Carboniferous arc magmatism preserved in the Chicxulub
impact structure

Catherine H. Ross; Daniel F. Stockli; Cornelia Rasmussen; Sean P.S. Gulick;
Sietze J. de Graaff …

Determining the nature and age of the 200-km-wide Chicxulub impact target
rock is an essential step in advancing our understanding of the Maya Block
basement. Few age constraints exist for the northern Maya Block crust,
specifically the basement underlying the 66 Ma, 200 km-wide Chicxulub
impact structure. The International Ocean Discovery Program-International
Continental Scientific Drilling Program Expedition 364 core recovered a
continuous section of basement rocks from the Chicxulub target rocks, which
provides a unique opportunity to illuminate the pre-impact tectonic
evolution of a terrane key to the development of the Gulf of Mexico. Sparse
published ages for the Maya Block point to Mesoproterozoic, Ediacaran,
Ordovician to Devonian crust are consistent with plate reconstruction
models. In contrast, granitic basement recovered from the Chicxulub peak
ring during Expedition 364 yielded new zircon U-Pb laser
ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)
concordant dates clustering around 334 ± 2.3 Ma. Zircon rare earth element
(REE) chemistry is consistent with the granitoids having formed in a
continental arc setting. Inherited zircon grains fall into three groups:
400−435 Ma, 500−635 Ma, and 940−1400 Ma, which are consistent with the
incorporation of Peri-Gondwanan, Pan-African, and Grenvillian crust,
respectively. Carboniferous U-Pb ages, trace element compositions, and
inherited zircon grains indicate a pre-collisional continental volcanic arc
located along the Maya Block’s northern margin before NW Gondwana collided
with Laurentia. The existence of a continental arc along NW Gondwana
suggests southward-directed subduction of Rheic oceanic crust beneath the
Maya Block and is similar to evidence for a continental arc along the
northern margin of Gondwana that is documented in the Suwannee terrane,
Florida, USA, and Coahuila Block of NE México.

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Detrital zircons from Late Paleozoic Ice Age sequences in Victoria Land
(Antarctica): New constraints on the glaciation of southern Gondwana

Luca Zurli; Gianluca Cornamusini; Jusun Woo; Giovanni Pio Liberato;
Seunghee Han …

The Lower Permian tillites of the Beacon Supergroup, cropping out in
Victoria Land (Antarctica), record climatic history during one of the
Earth’s coldest periods: the Late Paleozoic Ice Age. Reconstruction of
ice-extent and paleo-flow directions, as well as geochronological and
petrographic data, are poorly constrained in this sector of Gondwana. Here,
we provide the first detrital zircon U-Pb age analyses of both the Metschel
Tillite in southern Victoria Land and some tillites correlatable with the
Lanterman Formation in northern Victoria Land to identify the source
regions of these glaciogenic deposits. Six-hundred detrital zircon grains
from four diamictite samples were analyzed using laser ablation−inductively
coupled plasma−mass spectrometry. Geochronological and petrographic
compositional data of the Metschel Tillite indicate a widespread reworking
of older Devonian Beacon Supergroup sedimentary strata, with minor
contribution from Cambro-Ordovician granitoids and meta-sedimentary units
as well as Neoproterozoic metamorphic rocks. Euhedral to subhedral
Carboniferous−Devonian zircon grains match coeval magmatic units of
northern Victoria Land and Marie Byrd Land. This implies, in accordance
with published paleo-ice directions, a provenance from the east-southeast
sectors. In contrast, the two samples from northern Victoria Land tillite
reflect the local basement provenance; their geochronological age and
petrographic composition indicates a restricted catchment area with
multiple ice centers. This shows that numerous ice centers were present in
southern Gondwana during the Late Paleozoic Ice Age. While northern
Victoria Land hosted discrete glaciers closely linked with the northern
Victoria Land-Tasmania ice cap, the west-northwestward flowing southern
Victoria Land ice cap contributed most of the sediments comprising the
Metschel Tillite.

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Generation of Cretaceous high-silica granite by complementary crystal
accumulation and silicic melt extraction in the coastal region of
southeastern China

Jing-Yuan Chen; Jin-Hui Yang; Ji-Heng Zhang; Jin-Feng Sun; Yu-Sheng Zhu …

It is generally hypothesized that high-silica (SiO2 > 75 wt%)
granite (HSG) originates from crystal fractionation in the shallow crust.
Yet, identifying the complementary cumulate residue of HSG within plutons
remains difficult. In this work, we examine the genetic links between the
porphyritic monzogranite and HSG (including porphyritic granite,
monzogranite, and alkali feldspar granite) from the coastal area of
southeastern China using detailed zircon U-Pb ages, trace elements, Hf-O
isotopes, and whole-rock geochemistry and Nd-Hf isotopic compositions.
Zircon U-Pb ages indicate that the porphyritic monzogranite and HSG are
coeval (ca. 96−99 Ma). The HSG and porphyritic monzogranite have similar
formation ages within analytic error, identical mineral assemblages,
similar Nd-Hf isotopic compositions, and consistent variations in their
zircon compositions (i.e., Eu/Eu*, Zr/Hf, and Sm/Yb), which suggests that
their parental magma came from a common silicic magma reservoir and that
the lithological differences are the result of melt extraction processes.
The porphyritic monzogranite has relatively high SiO2 (70.0−73.4
wt%), Ba (718−1070 ppm), and Sr (493−657 ppm) contents, low K2O
and Rb concentrations and low Rb/Sr ratios (0.1−0.2), and it displays weak
Eu anomalies (Eu/Eu* = 0.57−0.90). Together with the petrographic features
of the porphyritic monzogranite, these geochemical variations indicate that
the porphyritic monzongranite is the residual silicic cumulate of the
crystal mush column. The HSG (SiO2 = 75.0−78.4) has variable
Rb/Sr ratios (2−490) and very low Sr (1−109 ppm) and Ba (9−323 ppm)
contents. Zircon from the HSG and porphyritic monzogranite overlap in
Eu/Eu*, Zr/Hf, and Sm/Yb ratios and Hf contents; however, some zircon from
the HSG show very low Eu/Eu* (0.1) and Zr/Hf ratios. These features
suggest that the HSG represents the high-silica melt that was extracted
from a crystal-rich mush. The injection of mantle-derived hotter mafic
magma into the mush column and the exsolution of F/Cl-enriched
volatiles (or fluids) from the interstitial melt rejuvenated the
pre-existing highly crystalline mush. Subsequent extraction and upward
migration of silicic melt resulting from compaction of the mush column
formed the HSG at shallow crustal levels, which left the complementary
crystal residue solidified as porphyritic monzogranite at the bottom.

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A newly discovered Late Cretaceous metamorphic belt along the active
continental margin of the Neo-Tethys ocean

Dan Wang; Fu-Lai Liu; Richard Palin; Jia-Min Wang; Mathias Wolf …

High-grade metamorphic rocks and crustal melts provide crucial evidence for
growth and differentiation of the continental crust, and are widespread in
collisional orogens. However, their importance in the evolution of
continental arcs remains poorly understood. Metamorphism and related
anatexis in the preserved continental margin of the Neo-Tethys ocean serves
as a key natural laboratory to investigate this process. Along the
Neo-Tethyan arc margin, the Gaoligong shear zone, Yunnan region of China,
is an important locality for linking Lhasa in the north with Sibumasu and
Burma in the south. Here, Late Cretaceous granulite-facies metamorphism and
crustal anatexis have been identified for the first time in the Gaoligong
area. Zircon and monazite U-Pb dating indicates that S-type granites formed
at 87−73 Ma, granites and buried pelitic sediments were simultaneously
metamorphosed at 75−70 Ma during Neo-Tethyan subduction, and all
lithologies were overprinted by a younger 40−30 Ma magmatic and strike-slip
event related to India-Asia collision. Phase equilibria modeling of
high-grade anatectic gneiss in the MnO-Na2O-CaO-K2
O-FeO-MgO-Al2O3-SiO2-H2O-TiO 2 system indicates peak pressure-temperature (P−T)
conditions of 780−800 °C and 6.5−7.5 kbar and defines a cooling and
decompressional P−T path for the metapelites. This demonstrates
that sediments within the Neo-Tethyan active continental arc were buried to
>20 km depth at 75−70 Ma. In combination with the metamorphic record of
the Lhasa, Burma, and Sibumasu blocks, an extensive Late Cretaceous
metamorphic belt must have formed along the Neo-Tethyan subduction zone.
This spatially correlates with coeval gabbro-diorite suites exposed in the
Gangdese, Sibumasu and Burma terranes that were triggered by thinning of
the lithospheric mantle. This prolonged Late Cretaceous mantle-derived
magmatism and lithospheric thinning may have provided a regional-scale heat
source for high-grade metamorphism and crustal anatexis along the active
continental margin of the Neo-Tethys ocean.

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Early Cenozoic partial melting of meta-sedimentary rocks of the eastern
Gangdese arc, southern Tibet, and its contribution to syn-collisional

Yuan-Yuan Jiang; Ze-Ming Zhang; Richard M. Palin; Hui-Xia Ding; Xuan-Xue Mo

Continental magmatic arcs are characterized by the accretion of voluminous
mantle-derived magmatic rocks and the growth of juvenile crust. However,
significant volumes of meta-sedimentary rocks occur in the middle and lower
arc crust, and the contributions of these rocks to the evolution of arc
crust remain unclear. In this paper, we conduct a systematic study of
petrology, geochronology, and geochemistry of migmatitic paragneisses from
the eastern Gangdese magmatic arc, southern Tibet. The results show that
the paragneisses were derived from late Carboniferous greywacke, and
underwent an early Cenozoic (69−41 Ma) upper amphibolite-facies
metamorphism and partial melting at pressure-temperature conditions of ∼11
kbar and ∼740 °C, and generated granitic melts with enriched Hf isotopic
compositions (anatectic zircon εHf(t) = −10.57 to +0.78).
Combined with the existing results, we conclude that the widely distributed
meta-sedimentary rocks in the eastern Gangdese arc deep crust have the same
protolith ages of late Carboniferous, and record northwestward-decreasing
metamorphic conditions. We consider that the deeply buried sedimentary
rocks resulted in the compositional change of juvenile lower crust from
mafic to felsic and the formation of syn-collisional S-type granitoids. The
mixing of melts derived from mantle, juvenile lower crust, and ancient
crustal materials resulted in the isotopic enrichment of the
syn-collisional arc-type magmatic rocks of the Gangdese arc. We suggest
that crustal shortening and underthrusting, and the accretion of
mantle-derived magma during the Indo-Asian collision transported the
supracrustal rocks to the deep crust of the Gangdese arc.

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Subduction initiation of the western Proto-Tethys Ocean: New evidence
from the Cambrian intra-oceanic forearc ophiolitic mélange in the
western Kunlun Orogen, NW Tibetan Plateau

Qichao Zhang; Zhong-Hai Li; Zhenhan Wu; Xuanhua Chen; Ji’en Zhang …

The supra-subduction zone ophiolite or ophiolitic mélange formed in the
forearc setting is generally considered to be a key geological record for
subduction initiation (SI) with petrological characteristics comparable to
the SI-related rock sequence from forearc basalt (FAB) to boninite in the
Izu-Bonin-Mariana subduction zone. Nevertheless, the standard FAB and
boninite are generally difficult to observe in the forearc rocks generated
during SI. Yet, a typical rock sequence indicating the SI of the western
Proto-Tethys Ocean is reported for the first time in the Qimanyute
intra-oceanic forearc system in the western Kunlun Orogen, Northwest
Tibetan Plateau. The magmatic compositions, which range from less to more
high field strength element (HFSE)-depleted and large ion lithophile
element (LILE)-enriched, are changing from oceanic plagiogranites (ca. 494
Ma) to forearc basalt-like gabbros (FAB-Gs, ca. 487 Ma), boninites, and
subsequent Nb-enriched gabbros (NEGs, ca. 485 Ma), which are thus
consistent with the Izu-Bonin-Mariana forearc rocks as well as the Troodos
and Semail supra-subduction zone-type ophiolites. The geochemical data from
the chemostratigraphic succession indicate a subduction initiation process
from a depleted mid-oceanic-ridge (MORB)-type mantle source with no
detectable subduction input to gradual increasing involvement of
subduction-derived materials (fluid/melts and sediments). The new
petrological, geochemical, and geochronological data, combined with the
regional geology, indicate that the well-sustained FAB-like intrusive
magmas with associated boninites could provide crucial evidence for SI and
further reveal that the SI of the western Proto-Tethys Ocean occurred in
the Late Cambrian (494−485 Ma).

View article:

Ancestral trans-North American Bell River system recorded in late
Oligocene to early Miocene sediments in the Labrador Sea and Canadian
Great Plains

Julia I. Corradino; Alex Pullen; Andrew L. Leier; David L. Barbeau Jr.;
Howie D. Scher …

The Bell River hypothesis proposes that an ancestral, transcontinental
river occupied much of northern North America during the Cenozoic Era,
transporting water and sediment from the North American Cordillera to the
Saglek Basin on the eastern margin of the Labrador Sea. To explore this
hypothesis and reconstruct Cenozoic North American drainage patterns, we
analyzed detrital zircon grains from the Oligocene−Miocene Mokami and
Saglek formations of the Saglek Basin and Oligocene−Miocene fluvial
conglomerates in the Great Plains of western Canada. U-Pb detrital zircon
age populations in the Mokami and Saglek formations include clusters at
250 Ma, 950−1250 Ma, 1600−2000 Ma, and 2400−3200 Ma. Detrital zircons
with ages of 250 Ma were derived from the North American Cordillera,
supporting the transcontinental Bell River hypothesis. Oligocene−Miocene
fluvial strata in western Canada contain detrital zircon age populations
similar to those in the Saglek Basin and are interpreted to represent the
western headwaters of the ancient Bell River drainage. Strontium-isotope
ratios of marine shell fragments from the Mokami and Saglek formations
yielded ages between 25.63 and 18.08 Ma. The same shells have εNd values of
−10.2 to −12.0 (average = −11.2), which are consistent with values of
Paleozoic strata in western North America but are more radiogenic than the
modern Labrador Current, Labrador Sea Water, and North Atlantic Deep Water
values (εNd ∼−12 to −25). As a freshwater source, the existence and
termination of the Bell River may have been important for Labrador Sea
circulation, stratification, and chemistry.

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Origin and age of the Shenshan tectonic mélange in the
Jiangshan-Shaoxing-Pingxiang Fault and late Early Paleozoic
juxtaposition of the Yangtze Block and the West Cathaysia terrane,
South China

Lijun Wang; Kexin Zhang; Shoufa Lin; Weihong He; Leiming Yin

When and how the Yangtze Block (Yangtze) and the West Cathaysia terrane
(West Cathaysia) in South China were amalgamated are critical to a better
understanding of the Neoproterozoic to early Paleozoic tectonic evolution
of South China and remain highly debatable. A key to this debate is the
tectonic significance of the Jiangshan-Shaoxing-Pingxiang (JSP) Fault, the
boundary between Yangtze and West Cathaysia. The Shenshan mélange along the
JSP Fault has the typical block-in-matrix structure and is composed of
numerous shear zone-bounded slivers/lenses of rocks of different types and
ages that formed in different tectonic environments, including middle to
late Tonian volcanic and volcanogenic sedimentary rocks (turbidite) of
arc/back-arc affinity, a series of middle Tonian ultramafic to mafic
plutonic rocks of oceanic island basalt affinity, a carbonaceous shale that
was deposited in a deep marine environment, and a red mudstone. U-Pb zircon
ages and acritarch assemblages (Leiosphaeridia-Brocholaminaria
association) found in the turbidite confirm its Tonian age, and fossils
from the carbonaceous shale (AsteridiumComasphaeridium
and Skiagia-CeltiberiumLeiofusa) constrains its age to
the Early to Middle Cambrian. Field relationships and available age data
leave no doubt that the ultramafic-mafic rocks are exotic blocks (rather
than intrusions) in the younger metasedimentary rocks. We conclude that the
Shenshan mélange is not an ophiolitic mélange, but rather a tectonic
mélange that formed as a result of movement along the JSP Fault in the
early Paleozoic. We suggest that Yangtze and West Cathaysia were two
separate microcontinents, were accreted to two different parts of the
northern margin of Gondwana in the early Early Paleozoic, and juxtaposed in
the late Early Paleozoic through strike-slip movement along the JSP Fault.
We further suggest that the ca. 820 Ma collision in the Jiangnan Orogen
took place between Yangtze and a (micro)continent that is now partly
preserved as the Huaiyu terrane and was not related to West Cathaysia. We
compare our model for South China with the accretion of terranes in the
North American Cordillera and propose a similar model for the relationship
between the Avalon and Meguma terranes in the Canadian Appalachians, i.e.,
the two terranes were accreted to two different parts of the Laurentian
margin and were later juxtaposed through margin-parallel strike slip

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