New GSA Bulletin Articles Published Ahead of Print in May
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; new
insights into the Chicxulub impact structure; and the dynamic topography of
the Cordilleran foreland basin. You can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Tectonic and eustatic control of Mesaverde Group
(Campanian–Maastrichtian) architecture, Wyoming-Utah-Colorado region,
USA
Keith P. Minor; Ronald J. Steel; Cornel Olariu
Abstract:
We describe and analyze the depositional history and stratigraphic
architecture of the Campanian and Maastrichtian succession of the southern
greater Green River basin of Wyoming, USA, and surrounding areas to better
understand the interplay between tectonic and eustatic drivers that built
the stratigraphy. By integrating new measured sections with published
outcrop, well-log, and paleogeographic data, two new stratigraphic
correlation diagrams, 35 new paleogeographic reconstructions, and six new
tectonic diagrams were created for this part of the Western Interior
Seaway. From this work, two time-scales of organization are evident: (1)
100−300 k.y.-scale, mainly eustatically driven regressive-transgressive
shoreline oscillations that generated repeated sequences of
alluvial-coastal plain-shoreline deposits, passing basinward to subaqueous
deltas, then capped by transgressive estuarine/barrier lagoon deposits, and
(2) 3.0−4.0 m.y.-scale, tectonically driven groups of 10 to 15 of these
eustatically driven units stacked in an offset arrangement to form larger
clastic units, which are herein referred to as clastic wedges. Four
regional clastic wedges are recognized, based on the architectures of these
clastic packages. These are the: (1) Adaville, (2) Rock Springs, (3) Iles,
and (4) Williams Fork clastic wedges. Pre-Mesaverde deposition in the
Wyoming-Utah-Colorado (USA) region during the Middle Cretaceous was
characterized by thickening of the clastic wedge close to the thrust-front,
driven primarily by retroarc foreland basin (flexural) tectonics. However,
a basinward shift in deposition during the Santonian into the early
Campanian (Adaville clastic wedge) signaled a change in the dominant
stratigraphic drivers in the region. Shoreline advance accelerated in the
early to middle Campanian (Rock Springs clastic wedge), as the end of
activity in the thrust belt, growing importance of flat-slab subduction,
and steady eastward migration of the zone of dynamic subsidence led to loss
of the foredeep and forebulge, with the attendant formation of a
low-accommodation shelf environment. This “flat-shelf” environment promoted
large shoreline advances and retreats during sea-level rise and fall.
During the middle to late Campanian (Iles clastic wedge), deep erosion on
the crest of the Moxa Arch, thinning on the crests of the Rock Springs and
Rawlins uplifts, and subsequent Laramide-driven basin formation occurred as
the Laramide blocks began to partition the region. The next clastic package
(Williams Fork clastic wedge) pushed the shoreline over 400 km away from
the thrust belt during the late Campanian. This was followed by a very
large and persistent marine transgression across the region, with the
formation of a Laramide-driven deepwater turbidite basin with toe-of-slope
fans into the early Maastrichtian. The Mesaverde Group in the
Wyoming-Utah-Colorado region is thus characterized by: (1) a succession of
four tectonically driven classic wedges, each comprised of a dozen or so
eustatically driven packages that preserve large basinward and landward
shoreline shifts, (2) broad regional sand and silt dispersal on a
low-accommodation marine shelf setting, (3) a progressive, tectonically
driven, basinward shift of deposition with offset, basinward stacking of
successive clastic wedges, and (4) the gradual formation of various uplifts
and sub-basins, the timing and sizes of which were controlled by the
movement of deep-seated Laramide blocks. The Mesaverde Group in the
Wyoming-Utah-Colorado region provides an outstanding opportunity to study
the dynamic interaction among the tectonic control elements of a subducting
plate (crustal loading-flexure, dynamic subsidence/uplift, and regional
flat-slab basin partitioning), as well as the dynamic interaction of
tectonic and eustatic controls.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36032.1/598755/Tectonic-and-eustatic-control-of-Mesaverde-Group
A new K-Ar illite dating application to constrain the timing of
subduction in West Sarawak, Borneo
Qi Zhao; Yi Yan; Satoshi Tonai; Naotaka Tomioka; Peter D. Clift ...
Abstract:
The timing of subduction is a fundamental tectonic problem for tectonic
models, yet there are few direct geological proxies for constraining it.
However, the matrix of a tectonic mélange formed in a subduction-accretion
setting archives the physical/chemical attributes at the time of
deformation during the subduction-accretion process. Thus, the deformation
age of the matrix offers the possibility to directly constrain the period
of the subduction-accretion process. Here we date the Lubok Antu tectonic
mélange and the overlying Lupar Formation in West Sarawak, Borneo by K-Ar
analysis of illite. The ages of authigenic illite cluster around 60 Ma and
36 Ma. The maximum temperatures calculated by vitrinite reflectance values
suggest that our dating results were not affected by external heating.
Thus, the ages of authigenic illite represent the deformation age of the
mélange matrix and the timing of the Rajang Unconformity, indicating that
the subduction in Sarawak could have continued until ca. 60 Ma and the
thermal and/or fluid flow events triggered by a major uplift of the Rajang
Group occurred at ca. 36 Ma. Furthermore, this study highlights the
potential of using the tectonic mélange to extract the timeframe of
subduction zone episodic evolution directly.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35895.1/598747/A-new-K-Ar-illite-dating-application-to-constrain
Constraining the effects of dynamic topography on the development of
Late Cretaceous Cordilleran foreland basin, western United States
Zhiyang Li; Jennifer Aschoff
Abstract:
Dynamic topography refers to the vertical deflection (i.e., uplift and
subsidence) of the Earth’s surface generated in response to mantle flow.
Although dynamic subsidence has been increasingly invoked to explain the
subsidence and migration of depocenters in the Late Cretaceous North
American Cordilleran foreland basin (CFB), it remains a challenging task to
discriminate the effects of dynamic mantle processes from other subsidence
mechanisms, and the spatial and temporal scales of dynamic topography is
not well known. To unravel the relationship between sedimentary systems,
accommodation, and subsidence mechanisms of the CFB through time and space,
a high-resolution chronostratigraphic framework was developed for the Upper
Cretaceous strata based on a dense data set integrating >600 well logs
from multiple basins/regions in Wyoming, Utah, Colorado, and New Mexico,
USA. The newly developed stratigraphic framework divides the Upper
Cretaceous strata into four chronostratigraphic packages separated by
chronostratigraphic surfaces that can be correlated regionally and
constrained by ammonite biozones. Regional isopach patterns and shoreline
trends constructed for successive time intervals suggest that dynamic
subsidence influenced accommodation creation in the CFB starting from ca.
85 Ma, and this wave of subsidence increasingly affected the CFB by ca. 80
Ma as subsidence migrated from the southwest to northeast. During 100−75
Ma, the depocenter migrated from central Utah (dominantly flexural
subsidence) to north-central Colorado (dominantly dynamic subsidence).
Subsidence within the CFB during 75−66 Ma was controlled by the combined
effects of flexural subsidence induced by local Laramide uplifts and
dynamic subsidence. Results from this study provide new constraints on the
spatio-temporal footprint and migration of large-scale (>400 km × 400
km) dynamic topography at an average rate ranging from ∼120 to 60 km/m.y.
in the CFB through the Late Cretaceous. The wavelength and location of
dynamic topography (subsidence and uplift) generated in response to the
subduction of the conjugate Shatsky Rise highly varied through both space
and time, probably depending on the evolution of the oceanic plateau (e.g.,
changes in its location, subduction angle and depth, and buoyancy).
Careful, high-resolution reconstruction of regional stratigraphic
frameworks using three-dimensional data sets is critical to constrain the
influence of dynamic topography. The highly transitory effects of dynamic
topography need to be incorporated into future foreland basin models to
better reconstruct and predict the formation of foreland basins that may
have formed under the combined influence of upper crustal flexural loading
and dynamic subcrustal loading associated with large-scale mantle flows.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35838.1/598220/Constraining-the-effects-of-dynamic-topography-on
Mid-Cretaceous thick carbonate accumulation in Northern Lhasa (Tibet):
eustatic vs. tectonic control?
Yiwei Xu; Xiumian Hu; Eduardo Garzanti; Marcelle BouDagher-Fadel; Gaoyuan
Sun ...
Abstract:
Widespread accumulation of thick carbonates is not typical of orogenic
settings. During the mid-Cretaceous, near the Bangong suture in the
northern Lhasa terrane, the shallow-marine carbonates of the Langshan
Formation, reaching a thickness up to ∼1 km, accumulated in an
epicontinental seaway over a modern area of 132 × 103 km 2, about half of the Arabian/Persian Gulf. The origin of
basin-wide carbonate deposits located close to a newly formed orogenic belt
is not well understood, partly because of the scarcity of paleogeographic
studies on the evolution of the northern Lhasa. Based on a detailed
sedimentological and stratigraphic investigation, three stages in the
mid-Cretaceous paleogeographic evolution of northern Lhasa were defined:
(1) remnant clastic sea with deposition of Duoni/Duba formations (Early to
early Late Aptian, ca. 125−116 Ma); (2) expanding carbonate seaway of
Langshan Formation (latest Aptian−earliest Cenomanian, ca. 116−99 Ma); and
(3) closure of the carbonate seaway represented by the Daxiong/Jingzhushan
formations (Early Cenomanian to Turonian, ca. 99−92 Ma). Combined with data
on tectonic subsidence and eustatic curves, we emphasized the largely
eustatic control on the paleogeographic evolution of the northern Lhasa
during the latest Aptian−earliest Cenomanian when the Langshan carbonates
accumulated, modulated by long-term slow tectonic subsidence and high
carbonate productivity.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35930.1/598221/Mid-Cretaceous-thick-carbonate-accumulation-in
Early and middle Miocene ice sheet dynamics in the Ross Sea: Results
from integrated core-log-seismic interpretation
Lara F. Pérez; Laura De Santis; Robert M. McKay; Robert D. Larter; Jeanine
Ash ...
Abstract:
Oscillations in ice sheet extent during early and middle Miocene are
intermittently preserved in the sedimentary record from the Antarctic
continental shelf, with widespread erosion occurring during major ice sheet
advances, and open marine deposition during times of ice sheet retreat.
Data from seismic reflection surveys and drill sites from Deep Sea Drilling
Project Leg 28 and International Ocean Discovery Program Expedition 374,
located across the present-day middle continental shelf of the central Ross
Sea (Antarctica), indicate the presence of expanded early to middle Miocene
sedimentary sections. These include the Miocene climate optimum (MCO ca.
17−14.6 Ma) and the middle Miocene climate transition (MMCT ca. 14.6−13.9
Ma). Here, we correlate drill core records, wireline logs and reflection
seismic data to elucidate the depositional architecture of the continental
shelf and reconstruct the evolution and variability of dynamic ice sheets
in the Ross Sea during the Miocene. Drill-site data are used to constrain
seismic isopach maps that document the evolution of different ice sheets
and ice caps which influenced sedimentary processes in the Ross Sea through
the early to middle Miocene. In the early Miocene, periods of localized
advance of the ice margin are revealed by the formation of thick sediment
wedges prograding into the basins. At this time, morainal bank complexes
are distinguished along the basin margins suggesting sediment supply
derived from marine-terminating glaciers. During the MCO,
biosiliceous-bearing sediments are regionally mapped within the depocenters
of the major sedimentary basin across the Ross Sea, indicative of
widespread open marine deposition with reduced glacimarine influence. At
the MMCT, a distinct erosive surface is interpreted as representing
large-scale marine-based ice sheet advance over most of the Ross Sea
paleo-continental shelf. The regional mapping of the seismic stratigraphic
architecture and its correlation to drilling data indicate a regional
transition through the Miocene from growth of ice caps and inland ice
sheets with marine-terminating margins, to widespread marine-based ice
sheets extending across the outer continental shelf in the Ross Sea.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35814.1/598222/Early-and-middle-Miocene-ice-sheet-dynamics-in-the
Late Quaternary aggradation and incision in the headwaters of the
Yangtze River, eastern Tibetan Plateau, China
Yang Yu; Xianyan Wang; Shuangwen Yi; Xiaodong Miao; Jef Vandenberghe ...
Abstract:
River aggradation or incision at different spatial-temporal scales are
governed by tectonics, climate change, and surface processes which all
adjust the ratio of sediment load to transport capacity of a channel. But
how the river responds to differential tectonic and extreme climate events
in a catchment is still poorly understood. Here, we address this issue by
reconstructing the distribution, ages, and sedimentary process of fluvial
terraces in a tectonically active area and monsoonal environment in the
headwaters of the Yangtze River in the eastern Tibetan Plateau, China.
Field observations, topographic analyses, and optically stimulated
luminescence dating reveal a remarkable fluvial aggradation, followed by
terrace formations at elevations of 55−62 m (T7), 42−46 m (T6), 38 m (T5),
22−36 m (T4), 18 m (T3), 12−16 m (T2), and 2−6 m (T1) above the present
floodplain. Gravelly fluvial accumulation more than 62 m thick has been
dated prior to 24−19 ka. It is regarded as a response to cold climate
during the last glacial maximum. Subsequently, the strong monsoon
precipitation contributed to cycles of rapid incision and lateral erosion,
expressed as cut-in-fill terraces. The correlation of terraces suggests
that specific tectonic activity controls the spatial scale and geomorphic
characteristics of the terraces, while climate fluctuations determine the
valley filling, river incision and terrace formation. Debris and colluvial
sediments are frequently interbedded in fluvial sediment sequences,
illustrating the episodic, short-timescale blocking of the channel ca. 20
ka. This indicates the potential impact of extreme events on geomorphic
evolution in rugged terrain.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35983.1/596999/Late-Quaternary-aggradation-and-incision-in-the
Late Neoproterozoic to early Paleozoic paleogeographic position of the
Yangtze block and the change of tectonic setting in its northwestern
margin: Evidence from detrital zircon U-Pb ages and Hf isotopes of
sedimentary rocks
Bingshuang Zhao; Xiaoping Long; Jin Luo; Yunpeng Dong; Caiyun Lan ...
Abstract:
The crustal evolution of the Yangtze block and its tectonic affinity to
other continents of Rodinia and subsequent Gondwana have not been well
constrained. Here, we present new U-Pb ages and Hf isotopes of detrital
zircons from the late Neoproterozoic to early Paleozoic sedimentary rocks
in the northwestern margin of the Yangtze block to provide critical
constraints on their provenance and tectonic settings. The detrital zircons
of two late Neoproterozoic samples have a small range of ages (0.87−0.67
Ga) with a dominant age peak at 0.73 Ga, which were likely derived from the
Hannan-Micangshan arc in the northwestern margin of the Yangtze block. In
addition, the cumulative distribution curves from the difference between
the depositional age and the crystalline age (CA−DA) together with the
mostly positive εHf(t) values of these zircon crystals
(−6.8 to +10.7, ∼90% zircon grains with εHf[t]
> 0) suggest these samples were deposited in a convergent setting during
the late Neoproterozoic. In contrast, the Cambrian−Silurian sediments share
a similar detrital zircon age spectrum that is dominated by Grenvillian
ages (1.11−0.72 Ga), with minor late Paleoproterozoic (ca. 2.31−1.71 Ga),
Mesoarchean to Neoarchean (3.16−2.69 Ga), and latest Archean to early
Paleoproterozoic (2.57−2.38 Ga) populations, suggesting a significant
change in the sedimentary provenance and tectonic setting from a convergent
setting after the breakup of Rodinia to an extensional setting during the
assembly of Gondwana. However, the presence of abundant Grenvillian and
Neoarchean ages, along with their moderately to highly rounded shapes,
indicates a possible sedimentary provenance from exotic continental
terrane(s). Considering the potential source areas around the Yangtze block
when it was a part of Rodinia or Gondwana, we suggest that the source of
these early Paleozoic sediments had typical Gondwana affinities, such as
the Himalaya, north India, and Tarim, which is also supported by their
stratigraphic similarity, newly published paleomagnetic data, and
tectono-thermal events in the northern fragments of Gondwana. This implies
that after prolonged subduction in the Neoproterozoic, the northwestern
margin of the Yangtze block began to be incorporated into the assembly of
Gondwana and then accept sediments from the northern margin of Gondwanaland
in a passive continental margin setting.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35980.1/597000/Late-Neoproterozoic-to-early-Paleozoic
Constraining the duration of the Tarim flood basalts (northwestern
China): CA-TIMS zircon U-Pb dating of tuffs
Yu-Ting Zhong; Zhen-Yu Luo; Roland Mundil; Xun Wei; Hai-Quan Liu ...
Abstract:
The Early Permian Tarim large igneous province (LIP) in northwestern China
comprises voluminous basaltic lava flows, as well as ultramafic and silicic
intrusions. The age and duration of the Tarim LIP remains unclear, and thus
the rate of magma production and models of potential environmental effects
are uncertain. Here we present high-precision chemical abrasion−isotope
dilution−thermal ionization mass spectrometry zircon U-Pb ages for three
newly discovered tuff layers interlayered with lava flows in the
Kupukuziman and Kaipaizileike formations in the Keping area (Xinjiang,
northwest China). The volcanism of the Kupukuziman Formation is constrained
to a short duration from 289.77 ± 0.95 to 289.41 ± 0.52 Ma. An age for the
overlying Kaipaizileike Formation is 284.27 ± 0.39 Ma, bracketing the
duration of the entire eruptive phase of the Tarim flood basalts at ∼5.5
m.y. The low eruption rate and relatively long duration of magmatism is
consistent with a plume incubation model for the Tarim LIP.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36053.1/597001/Constraining-the-duration-of-the-Tarim-flood
Late Pleistocene−Holocene flood history, flood-sediment provenance and
human imprints from the upper Indus River catchment, Ladakh Himalaya
Choudhurimayum Pankaj Sharma; Poonam Chahal; Anil Kumar; Saurabh Singhal;
YP Sundriyal ...
Abstract:
The Indus River, originating from Manasarovar Lake in Tibet, runs along the
Indus Tsangpo suture zone in Ladakh which separates the Tethyan Himalaya in
the south from the Karakoram zone to the north. Due to the barriers created
by the Pir-Panjal ranges and the High Himalaya, Ladakh is located in a rain
shadow zone of the Indian summer monsoon (ISM) making it a high-altitude
desert. Occasional catastrophic hydrological events are known to endanger
lives and properties of people residing there. Evidence of such events in
the recent geologic past that are larger in magnitude than modern
occurrences is preserved along the channels. Detailed investigation of
these archives is imperative to expand our knowledge of extreme floods that
rarely occur on the human timescale. Understanding the frequency,
distribution, and forcing mechanisms of past extreme floods of this region
is crucial to examine whether the causal agents are regional, global, or
both on long timescales. We studied the Holocene extreme flood history of
the Upper Indus catchment in Ladakh using slackwater deposits (SWDs)
preserved along the Indus and Zanskar Rivers. SWDs here are composed of
stacks of sand-silt couplets deposited rapidly during large flooding events
in areas where a sharp reduction of flow velocity is caused by local
geomorphic conditions. Each couplet represents a flood, the age of which is
constrained using optically stimulated luminescence for sand and
accelerator mass spectrometry and liquid scintillation counter 14C for charcoal specks from hearths. The study suggests
occurrence of large floods during phases of strengthened ISM when the
monsoon penetrated into arid Ladakh. Comparison with flood records of
rivers draining other regions of the Himalaya and those influenced by the
East Asian summer monsoon (EASM) indicates asynchronicity with the Western
Himalaya that confirms the existing anti-phase relationship of the ISM-EASM
that occurred in the Holocene. Detrital zircon provenance analysis
indicates that sediment transportation along the Zanskar River is more
efficient than the main Indus channel during extreme floods. Post−Last
Glacial Maximum human migration, during warm and wet climatic conditions,
into the arid upper Indus catchment is revealed from hearths found within
the SWDs.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35976.1/597002/Late-Pleistocene-Holocene-flood-history-flood
New insights into the formation and emplacement of impact melt rocks
within the Chicxulub impact structure, following the 2016 IODP-ICDP
Expedition 364
Sietze J. de Graaff; Pim Kaskes; Thomas Déhais; Steven Goderis; Vinciane
Debaille ...
Abstract:
This study presents petrographic and geochemical characterization of 46
pre-impact rocks and 32 impactites containing and/or representing impact
melt rock from the peak ring of the Chicxulub impact structure (Yucatán,
Mexico). The aims were both to investigate the components that potentially
contributed to the impact melt (i.e., the pre-impact lithologies) and to
better elucidate impact melt rock emplacement at Chicxulub. The impactites
presented here are subdivided into two sample groups: the lower impact melt
rock−bearing unit, which intrudes the peak ring at different intervals, and
the upper impact melt rock unit, which overlies the peak ring. The
geochemical characterization of five identified pre-impact lithologies
(i.e., granitoid, dolerite, dacite, felsite, and limestone) was able to
constrain the bulk geochemical composition of both impactite units. These
pre-impact lithologies thus likely represent the main constituent
lithologies that were involved in the formation of impact melt rock. In
general, the composition of both impactite units can be explained by mixing
of the primarily felsic and mafic lithologies, but with varying degrees of
carbonate dilution. It is assumed that the two units were initially part of
the same impact-produced melt, but discrete processes separated them during
crater formation. The lower impact melt rock−bearing unit is interpreted to
represent impact melt rock injected into the crystalline basement during
the compression/excavation stage of cratering. These impact melt rock
layers acted as delamination surfaces within the crystalline basement,
accommodating its displacement during peak ring formation. This movement
strongly comminuted the impact melt rock layers present in the peak ring
structure. The composition of the upper impact melt rock unit was
contingent on the entrainment of carbonate components and is interpreted to
have stayed at the surface during crater development. Its formation was not
finalized until the modification stage, when carbonate material would have
reentered the crater.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35795.1/597003/New-insights-into-the-formation-and-emplacement-of
Isotopic spatial-temporal evolution of magmatic rocks in the Gangdese
belt: Implications for the origin of Miocene post-collisional giant
porphyry deposits in southern Tibet
Chen-Hao Luo; Rui Wang; Roberto F. Weinberg; Zengqian Hou
Abstract:
Crustal growth is commonly associated with porphyry deposit formation
whether in continental arcs or collisional orogens. The Miocene high-K
calc-alkaline granitoids in the Gangdese belt in southern Tibet, associated
with porphyry copper deposits, are derived from the juvenile lower crust
with input from lithospheric mantle trachytic magmas, and are characterized
by adakitic affinity with high-Sr/Y and La/Yb ratios as well as high Mg#
and more evolved isotopic ratios. Researchers have argued, lower crust with
metal fertilization was mainly formed by previous subduction-related
modification. The issue is that the arc is composed of three stages of
magmatism including Jurassic, Cretaceous, and Paleocene−Eocene, with peaks
of activity at 200 Ma, 90 Ma, and ca. 50 Ma, respectively. All three stages
of arc growth are essentially similar in terms of their whole-rock
geochemistry and Sr-Nd-Hf isotopic compositions, making it difficult to
distinguish Miocene magma sources. This study is based on ∼430 bulk-rock
Sr-Nd isotope data and ∼270 zircon Lu-Hf isotope data and >800
whole-rock geochemistry analyses in a 900-km-long section of the Gangdese
belt. We found large scale variations along the length of the arc where the
Nd-Hf isotopic ratios of the Jurassic, Cretaceous, and Paleocene−Eocene arc
rocks change differently from east to west. A significant feature is that
the spatial distribution of Nd-Hf isotopic values of the Paleocene−Eocene
arc magmas and the Miocene granitoids, including metallogenic ones, are
“bell-shaped” from east to west, with a peak of εNd(t) and εHf (t) at ∼91°E. In contrast, the Jurassic and Cretaceous arc
magmas have different isotopic distribution patterns as a function of
longitude. The isotopic spatial similarity of the Paleocene−Eocene and
Miocene suites suggests that the lower crust source of the metallogenic
Miocene magmas is composed dominantly of the Paleocene−Eocene arc rocks.
This is further supported by abundant inherited zircons dominated by
Paleocene−Eocene ages in the Miocene rocks. Another important discovery
from the large data set is that the Miocene magmatic rocks have higher Mg # and more evolved Sr-Nd-Hf isotopic compositions than all
preceding magmatic arcs. These characteristics indicate that the
involvement of another different source was required to form the Miocene
magmatic rocks. Hybridization of the isotopically unevolved primary magmas
with isotopically evolved, lithospheric mantle-derived trachytic magmas is
consistent with the geochemical, xenolith, and seismic evidence and is
essential for the Miocene crustal growth and porphyry deposit formation. We
recognize that the crustal growth in the collisional orogen is a two-step
process, the first is the subduction stage dominated by typical magmatic
arc processes leading to lower crust fertilization, the second is the
collisional stage dominated by partial melting of a subduction-modified
lower crust and mixing with a lithospheric mantle-derived melt at the
source depth.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36018.1/596769/Isotopic-spatial-temporal-evolution-of-magmatic
Oxygen isotope (δ18O) trends measured from Ordovician
conodont apatite using secondary ion mass spectrometry (SIMS):
Implications for paleo-thermometry studies
Cole T. Edwards; Clive M. Jones; Page C. Quinton; David A. Fike
Abstract:
The oxygen isotopic compositions (δ18O) of minimally altered phosphate
minerals and fossils, such as conodont elements, are used as a proxy for
past ocean temperature. Phosphate is thermally stable under low to moderate
burial conditions and is ideal for reconstructing seawater temperatures
because the P-O bonds are highly resistant to isotopic exchange during
diagenesis. Traditional bulk methods used to measure conodont δ18O include
multiple conodont elements, which can reflect different environments and
potentially yield an aggregate δ18O value derived from a mixture of
different water masses. In situ spot analyses of individual elements using
micro-analytical techniques, such as secondary ion mass spectrometry
(SIMS), can address these issues. Here we present 108 new δ18O values using
SIMS from conodont apatite collected from four Lower to Upper Ordovician
stratigraphic successions from North America (Nevada, Oklahoma, and the
Cincinnati Arch region of Kentucky and Indiana, USA). The available
elements measured had a range of thermal alteration regimes that are
categorized based on their conodont alteration index (CAI) as either low
(CAI = 1−2) or high (CAI = 3−4). Though individual spot analyses of the
same element yield δ18O values that vary by several per mil (‰), most form
a normal distribution around a mean value. Isotopic variability of
individual spots can be minimized by avoiding surficial heterogeneities
like cracks, pits, or near the edge of the element and the precision can be
improved with multiple (≥4) spot analyses of the same element. Mean δ18O
values from multiple conodonts from the same bed range between 0.0 and 4.3‰
(median 1.0‰), regardless of low or high CAI values. Oxygen isotopic values
measured using SIMS in this study reproduce values similar to published
trends, namely, δ18O values increase during the Early−Middle Ordovician and
plateau by the mid Darriwilian (late Middle Ordovician). Twenty-two of the
measured conodonts were from ten sampled beds that had been previously
measured using bulk analysis. SIMS-based δ18O values from these samples are
more positive by an average of 1.7‰ compared to bulk values, consistent
with observations by others who attribute the shift to carbonate- and
hydroxyl-related SIMS matrix effects. This offset has implications for
paleo-temperature model estimates, which indicate that a 4 °C temperature
change corresponds to a 1‰ shift in δ18O (‰). Although this uncertainty
precludes precise paleo-temperature reconstructions by SIMS, it is valuable
for identifying spatial and stratigraphic trends in temperature that might
not have been previously possible with bulk approaches.
View article
:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35891.1/596655/Oxygen-isotope-18O-trends-measured-from-Ordovician
GSA BULLETIN articles published ahead of print are online at
https://bulletin.geoscienceworld.org/content/early/recent
. Representatives of the media may obtain complimentary copies of articles
by contacting Kea Giles. Please discuss articles of interest with the
authors before publishing stories on their work, and please make reference
to The Geological Society of America Bulletin in articles published.
Non-media requests for articles may be directed to GSA Sales and Service,
gsaservice@geosociety.org.
https://www.geosociety.org
# # #