Anthony I.S. Kemp

A genetic linkage between subduction- and collision-related porphyry Cu deposits in continental collision zones

The genesis of continental collision-related porphyry Cu deposits (PCDs) remains controversial. The most common hypothesis links their genesis with magmas derived from subduction-modified arc lithosphere. However, it is unclear whether a genetic linkage exists between collision- and subduction-related PCDs. Here, we studied Jurassic subduction-related Cu-Au and Miocene collision-related Cu-Mo porphyry deposits in south Tibet. The Jurassic PCDs occur only in the western segment of the Jurassic arc, which has depleted mantle-like isotopic compositions [e.g., ( 87 Sr/ 86 Sr) i = 0.7041–0.7048; e Nd(t) as high as 7.5, and e Hf(t) as high as 18]. By contrast, no Jurassic PCDs have been found in the eastern arc segment, which is isotopically less juvenile [e.g., ( 87 Sr/ 86 Sr) i = 0.7041–0.7063, e Nd(t) < 4.5, and e Hf(t) ≤ 12]. These results imply that incorporation of crustal components during underplating of Jurassic magma induced copper accumulation as sulfides at the base of the eastern Jurassic arc, inhibiting PCD formation at this time. Miocene PCDs are spatially confined to the Jurassic arc, and the giant Miocene PCDs cluster in its eastern segment where no Jurassic PCDs occur. This suggests that the arc segment barren for subduction-related PCDs could be fertile for collision-related PCDs. Miocene ore-forming porphyries have young Hf model ages and Sr-Nd-Hf isotopic compositions overlapping with those of the Jurassic rocks in the eastern segment, whereas contemporaneous barren porphyries outside the Jurassic arc have abundant zircon inheritance and crustlike Sr-Nd-Hf isotopic compositions. These data suggest that remelting of the lower crustal sulfide-bearing Cu-rich Jurassic cumulates, triggered by Cenozoic crustal thickening and/or subsequent slab break-off, led to the formation of giant Miocene PCDs. The spatial overlap and complementary metal endowment between subduction- and collision-related magmas may be used to evaluate the mineral potential for such deposits in other orogenic belts.

Age, nature, and origin of Ordovician Zhibenshan granite from the Baoshan terrane in the Sanjiang region and its significance for understanding Proto-Tethys evolution

This contribution reports on a better understanding of the Proto-Tethys evolution in the Sanjiang Tethyan region of China. The manuscript presents laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon ages, Sr–Nd–Hf isotope systematics, and whole-rock major and trace element of Ordovician magmatic rocks from the calc-alkaline Zhibenshan granite, which formed along the northern margin of East Gondwana. The U–Pb zircon dating of monzogranite from the Zhibenshan granite yields crystallization ages of 466–457 Ma. The monzogranite has Cross–Iddings–Pirsson–Washington normative corundum (3.34%) and is peraluminous with Al2O3/(Na2O+ K2O+CaO) molar ratio of 1.26, similar to S-type granites. All samples are enriched in large ion lithophile elements (LILEs, such as Rb, K, U, and Th) and depleted in high field strength elements (HFSEs, e.g. Nb and Ti). These granites are enriched in light rare earth elements (REEs) and depleted in heavy REEs with strongly negative Eu anomalies (δEu = 0.19–0.24). The initial 87Sr/86Sr ratios range from 0.7118 to 0.7176 and εNd(t) values from −11.3 to −10.3 with Nd model ages of 2114–2037 Ma. Magmatic zircons with early Palaeozoic dates have εHf(t) values ranging from −13.3 to −1.8 and Hf model ages from 2258 to 1537 Ma. These geochemical and isotopic features suggest that the Zhibenshan granite originated from an ancient crustal source. Ordovician granites in the Baoshan terrane represent the southward continuation of the early Palaeozoic granitic belt that extended along the northern margin of East Gondwana, providing important evidence for the evolution of the Proto-Tethyan Ocean.

Large igneous province or long-lived magmatic arc along the eastern margin of Australia during the Cretaceous? Insights from the sedimentary record

U-Pb geochronology and Lu-Hf isotope analysis of detrital zircon from the mid-Cretaceous Winton and Mackunda Formations in the Eromanga Basin were employed to investigate regional provenance patterns in order to better understand the tectonic setting and paleogeography of eastern Australia during the late Mesozoic. A suite of Mesozoic-aged zircon populations recovered from these formations suggests that volcanism along the eastern margin of Australia was relatively continuous from the Triassic (252 Ma) to at least the mid-Cretaceous (ca. 92 Ma). Cretaceous-age zircon populations dominate the provenance record, and a distinct upsection younging trend in Cretaceous grain ages indicates that deposition was largely synchronous with ongoing volcanism to the east. Lu-Hf isotopic data suggest that these zircon populations were sourced from igneous rocks of a mixed juvenile and crustal source, similar to Lu-Hf isotopic systematics for eastern Australian zircons from Pennsylvanian–Permian igneous assemblages (307–252 Ma), for which an active convergent margin association is well established. An extensive Cretaceous volcanic terrain, now limited to the Whitsunday Igneous Association, was once located along the northeastern margin of Australia. Results from this study support the hypothesis that the Whitsunday igneous association was the main source of Cretaceous sediment to the Eromanga Basin, and likely for sediment transported across the continent southward and into the Ceduna Delta system offshore South Australia. The Whitsunday igneous association has been interpreted as a siliceous large igneous province associated with the onset of rifting in the region and linked to opening of the Tasman and Coral Seas. Yet, in this study, we document a relatively continuous Late Triassic to Late Cretaceous (240–92 Ma) age range for detrital zircons from the Mackunda and Winton Formations, consistent with relatively uninterrupted magmatic activity along the continental margin until ca. 92 Ma (earliest Turonian). Furthermore, zircon grains across this age spectrum exhibit dominantly positive to strongly positive eHf(t) values, between +4 and +12, consistent with values known for zircon suites from older magmatic arc rocks of eastern Australia. Although these data do not support a conclusive interpretation, they are consistent with an east Australian magmatic arc related to westward subduction of paleo-Pacific oceanic crust beneath eastern Australia enduring into the Cretaceous, as distinct from extensional siliceous large igneous province magmatism unrelated to subduction and generated by rupture of continental crust.

New Zircon U-Pb and Hf-isotope data of the Birimian Terrane of the West African craton

Oceanic anoxic events (OAEs) were a frequent occurrence in the Cretaceous greenhouse ocean. Based on a variety of paleoredox indicators, euxinic water column conditions are commonly invoked for these OAEs. However, in a high resolution study of OAE3 deep sea sediments [1], revised paleoredox indicators suggest that euxinic conditions fluctuated with anoxic ferruginous conditions on orbital timescales. Building upon this, we here present new data for a continental shelf setting at Tarfaya, Morocco, that spans a period prior to, and during, the onset of OAE2. We again find strong evidence for orbital transitions from euxinic to ferruginous conditions. The presence of this distinct cyclicity during OAE2 and OAE3 in shallow and deep water settings, coupled with its occurrence on the anoxic shelf prior to the global onset of anoxia, suggests that these fluctuations were a fundamental feature of anoxia in the Cretaceous ocean. The observed redox cyclicity has major implications for the cycling of phosphorus, and hence the maintenance and longevity of OAEs. However, despite this significance, controls on the observed redox cyclicity are essentially unknown. Here, we utilize S isotope measurements (pyrite S and carbonate-associated S) from the deep sea and shelf settings to model oceanic sulphate concentrations across the redox transitions. Perhaps surprisingly, we find no evidence to suggest that ferruginous conditions arose due to extensive drawdown of seawater sulphate (as pyrite-S and organic-S) under euxinic conditions. Instead, S isotope systematics in the deep sea imply increased sulphate concentrations during ferruginous intervals. Based on these observations and other major element data, we infer that the redox cyclicity instead relates to orbitally-paced fluctuations in continental hydrology and weathering, linking the redox state of the global ocean to climate-driven processes on land. [1] Marz et al (2008) GCA, 72, 3703-3717.

Early earth geodynamics: cross examining the geological testimony

Many studies link the presence of continents on Earth to the operation of plate tectonics. Radiogenic isotope data have, however, long consigned the bulk of crust generation and preservation to the murky realm of the Precambrian Earth, where the prevailing geodynamic systems are highly uncertain due to the sparse and complex nature of the geological record of these early eons. The purpose of this paper is to examine the nature of this geological record, considering the biases and artefacts that may undermine its fidelity, and to assess what are the most robust lines of evidence from which meaningful geodynamic inferences can be drawn. This is pursued with reference to Hadean detrital zircons, Archean gneiss complexes and Archean granite–greenstone terranes, and by considering isotopic proxies of crust–mantle interaction. The evidence reinforces long held views that the formation of some of the oldest continental nuclei involved a distinctive mode of planetary geodynamics that rests uneasily within definitions of modern style plate tectonics. A detailed interrogation of the oldest rocks, integrating multi-scale information from the best preserved whole-rock and mineral archives, and emphasizing careful selection at the sampling and analytical stages, will lead to the most robust input data for petrological and thermodynamic models of early Earth processes. This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics’.

Tetrahedral plot diagram: A geometrical solution for quaternary systems

Abstract The transformation from a tetrahedral four-component system to an XYZ-orthogonal coordinate axis system has been solved using the geometry of a tetrahedron. If a four component mixing ratio is described as t, l, r, and f (here, t + l + r + f = 1), the transforming equations can be written as A tetrahedral plot diagram can be easily constructed using the algorithms described in this paper. We present an implementation of these algorithms in a custom-designed Microsoft Excel spreadsheet, including adjustable viewing angles for the tetrahedral plot. This will be of general utility for petrological or mineralogical studies of quaternary systems.

Generation of I-type granitic rocks by melting of heterogeneous lower crust

[Extract]: Granite generation is a fundamental process for the growth and evolution of Earth’s continental crust. I-type granitic rocks, nominally derived from infracrustal sources, are the most common granite type and are voluminously emplaced in convergent margin settings. A puzzling feature is that many I-type granites show isotopic evidence for reworking of older supracrustal material, in conflict with the I-type designation. How the supracrustal component was incorporated by I-type magmas is a matter of deduction, particularly given difficulties in recognizing the putative infracrustal source region in the exposed geology. We report a case study of I-type granitic magma generation by hybridization between metasedimentary-derived partial melt and intercalated mafic granulite units during extraction of silicic magma from the lower crust in the Hidaka Metamorphic Belt (HMB), Japan (Hammerli et al., 2018). Isotopic data (Nd, Hf, O) obtained by microanalysis of accessory minerals in former melt networks (leucosomes) suggest that hybridization operates on a (sub-) grain scale, where repeated injections of externally derived melt attempt to approach local equilibrium with the host mafic granulites during transfer through complex melt pathways (see our figure 3; and also Hasalova et al. [2011] and references therein).

An Archean Yellowstone? Evidence from extremely low δ18O in zircons preserved in granulites of the Yilgarn Craton, Western Australia

We report the discovery of Archean (2980-2670 Ma) zircons from the Yilgarn Craton in Western Australia that record unusually low delta O-18 signatures (to -0.5%). These zircons occur in cordierite-orthopyroxene granulites that retain the geochemical signature of intense premetamorphic hydrothermal alteration. We propose a model whereby the low-delta O-18 zircons crystallized within protoliths that record multiple stages of high-temperature interaction and hydrothermal exchange between shallow crustal material and O-18-depleted meteoric fluids, in a setting analogous to that of the Yellowstone caldera. Burial and subsequent granulitefacies metamorphism of this crust led to the crystallization of zircon, which acquired and preserved the extremely O-18-depleted signature of the whole rock. The apparent absence of strongly O-18 depleted Archean zircons has been a puzzling feature of the global zircon record, but we suggest this is an artifact of poor preservation potential. Our findings suggest that long-lived, shallow crustal magmatic-hydrothermal systems similar to those operating in modern caldera complexes were also a feature of Archean Earth.

Hospitalization for drug‐induced hepatotoxicity: linking Y‐codes with pharmaceutical claims data to identify implicated medicines

Hospital admissions associated with an adverse drug reaction are often coded to the International Classification of Diseases external cause Y‐codes, denoting the medicine class deemed to cause the adverse drug reaction. Matching hospital data with outpatient dispensing data has the potential to identify the specific causative medicines but the ability to identify the causative medicines in this way has not been previously assessed. This study aimed to determine the proportion of Y‐coded hospitalizations for drug‐induced hepatotoxicity that could be matched with a potential causative medicine from outpatient dispensing data.