Neal J. McNaughton

First evidence of >3.2 Ga continental crust in the Yangtze craton of south China and its implications for Archean crustal evolution and Phanerozoic tectonics

Ion microprobe (SHRIMP II) U-Pb zircon analyses reveal trondhjemitic magmatism at 2.90–2.95 Ga in the Kongling area of the Yangtze craton, south China, about 150 km south of the Permian-Triassic Qinling-Dabie-Sulu orogenic belt. Detrital zircons from nearby Archean metapelites are 2.87–3.28 Ga, and the rocks have Sm-Nd depleted mantle model ages of 3.07–3.21 Ga. The new data reveal, for the first time, >3.2 Ga sialic crust in the Yangtze craton, part of which predates that of the adjacent southern North China craton. Both trondhjemites and metapelites contain ca. 2.75 Ga high-grade metamorphic zircons, and ca. 1.9 Ga zircons, related to intrusion of the Quanqitang K-feldspar granite into the Archean basement. Many zircons also underwent Pb loss ca. 1.0 Ga during the Jinning orogeny, when the Cathysian block accreted to the Yangtze craton. The new data support correlation of part of the Korean Peninsula with the Yangtze craton along the eastern extension of the Qinling-Dabie-Sulu orogenic belt.

Constraints on crustal evolution and gold metallogeny in the Northwestern Jiaodong Peninsula, China, from SHRIMP U–Pb zircon studies of granitoids

Abstract The northwestern part of the Jiaodong Peninsula, or the Zhao-Ye Gold Belt, contains the largest lode-gold deposits in China, which are spatially related to several suites of intrusive granitoids. Previous attempts to provide constraints on the timing of gold mineralization and its tectonic setting, through studies of the granitoids, have led to conflicting data, dependant on the isotopic methodology used, and resultant genetic and tectonic models for their setting are equivocal. SHRIMP U–Pb studies of complexly zoned zircons of the Linglong, Luanjiahe and Guojialing granitoid suites suggest that the Jiaodong Peninsula is underlain by Precambrian basement with components up to 3.4 Ga old. Inherited zircons of early Mesozoic age indicate that this basement was reworked at 250–200 Ma, probably during a collisional orogeny involving the North China Craton and South China Craton. The Linglong, Luanjiahe and Guojialing suites were derived from this early Mesozoic basement between 165 and 125 Ma, and were probably emplaced as post-collisional granitoids, the latest intrusions (at 125 Ma) coinciding broadly with superplume activity or a major plume breakout event in the Palaeo-Pacific Plate. Importantly, if lode-gold mineralization is essentially a single event, as indicated by similar hosting structures and deposit characteristics, it can be dated between about 126 Ma, the age of the youngest granitoid cut by gold-bearing quartz veins, and 120 Ma, the age of one of a swarm of post-mineralization feldspar-porphyry dykes. Thus, as in most other metallogenic provinces which host so-called mesothermal lode-gold deposits, gold mineralization was late in the orogenic cycle, probably late- or post-accretion, and closely followed the emplacement of the latest major plutonic phase in the development of a series of anomalously voluminous granitoid batholiths.

The Sholl Shear Zone, West Pilbara: evidence for a domain boundary structure from integrated tectonostratigraphic analyses, SHRIMP UPb dating and isotopic and geochemical data of granitoids

Abstract The Pilbara Block provides a record of Archaean continental growth involving the tectonic accretion of outboard island-arcs and collisions with other continental-scale fragments. This record of continental growth is balanced by breakup and strike-slip dismemberment of the continent. New SHRIMP UPb in zircon ages and SmNd data provide evidence in the West Pilbara which demonstrates that subduction-related and tectonic-accretion processes at the western margin of that ancestral continent between 3.15-2.78 Ga were coeval with, and genetically related to, crustal-scale tectonics and basin formation inboard of that margin. The tectonic division of the West Pilbara is defined by integrated tectonic analyses, geochronology, geochemistry and isotopic analyses. Geochronological studies clearly indicate that the western Pilbara comparises two domains with different recorded geohistories, whereas geochemistry and isotopic systematics reflect the changing tectonic regimes through time. In combination, these studies allow the development of a reconstruction of the relative positions of the domains through time on the western margin of the Pilbara Block. The supracrustal rocks of the northern Roebourne Lithotectonic Complex (Domain 6 in a Pilbarawide scheme) were formed in an island arc setting, facing an ocean to the north-west, prior to 3260 Ma, the time of emplacement of voluminous granitoids into the complex. In contrast, the supracrustal rocks of the southern Sholl Belt (Pilbara Domain 5) were formed in a back-arc setting behind a north-west-facing arc between 3125 and 3112 Ma, with more-or-less synchronous granite emplacement at about 3115 Ma. The two domains were tectonically juxtaposed, between 2991 and 2925 Ma, by the Sholl Shear Zone, a largely sinistral shear zone, with subsequent volcanism in both domains to about 2925 Ma. The Roebourne Lithotectonic Complex (Domain 6) is interpreted to be an allochthonous terrane, which formed north-east relative to its present position, but indigenous to the Pilbara Block rather than an exotic terrane. The East Pilbara is interpreted to have acted as a cratonic hinterland during the convergent margin tectonics that affected the two West Pilbara domains.

1.6 Ga U-Pb zircon age for the Chorhat Sandstone, lower Vindhyan, India: Possible implications for early evolution of animals

Bedding-plane markings in the Chorhat Sandstone (lower Vindhyan), central India, were recently interpreted as burrows produced by triploblastic animals. Because the rocks were thought to be older than 1000 Ma, these structures were regarded as the oldest fossil evidence for metazoan life. However, the biological origin of the markings has been questioned, as has their age. Current age estimates are based on K-Ar, Rb-Sr, and fission- track dates, though some contentious evidence suggests that the rocks may be only 540 Ma. Here we provide the first robust age data for the lower Vindhyan by using SHRIMP (sensitive, high-resolution ion microprobe) U-Pb zircon geochronology to date silicified tuffs bounding the Chorhat Sandstone. Our results show that the sediments were deposited between 1628 ± 8 Ma and 1599 ± 8 Ma. If the Chorhat markings are burrows left by worm-like animals, then our data suggest that complex metazoans had evolved before 1600 Ma, 1 b.y. before the “Cambrian explosion” when animals rapidly diversified and became ecologically dominant. However, given the doubts expressed about the origin of the bedding-plane structures, as well as the surprisingly “old” age of the host rocks, further studies are urgently required to provide supportive evidence.

Advances in SHRIMP geochronology and their impact on understanding the tectonic and metallogenic evolution of southern Brazil

Significant improvements, both in understanding the evolution of zircons and in understanding the geotectonic and metallogenetic evolution of the complex terrain of southern Brazil, are obtained from a SHRIMP geochronology study and reviewed in this paper. The use of backscattered electron and cathodoluminescence images, prior to SHRIMP isotopic determinations, proved of enormous fundamental value for this technique. Zircon is a domainal open‐system mineral in many geological conditions; very old domains may be preserved, but the same crystal may show ages of younger tectonic events. Zircons may recrystallise inwards from the rims or outwards from the cores, and also along euhedral high‐U or metamict thin zones. Zircons also may be recrystallised during gold‐related hydrothermalism, phyllic alteration of granitic rocks. The precise dating of amphibolite dykes can be achieved by the identification and dating of magmatic zircons. Precambrian orogenies are identified along with the intervening intracratonic tectonic cycles of supercontinents in southern Brazil from 3300 to 470 Ma. Granulite protoliths were formed during the Jequié Orogeny (ca 2600 Ma), but extensive arc accretion occurred in the Palaeoproterozoic (ca 2250 Ma) Encantadas Orogeny. Late in the Transamazonian Cycle, granites were formed by crustal melting at about 2000 Ma in the Camboriú Orogeny. Both accretionary and collisional orogenies are also identified in the Neoproterozoic Brasiliano Cycle. These are the accretionary Passinho Orogeny (ca 900 Ma) and São Gabriel Orogeny (ca 700 Ma), that were succeeded by the collisional Dom Feliciano Orogeny (ca 600 Ma). Base‐metal and gold deposition occurred in juvenile island arcs and in late orogenic porphyry‐copper‐type magmatic‐hydrothermal settings during the Neoproterozoic.

Early Archean crust in Bastar Craton, Central India—a geochemical and isotopic study

Abstract High-A12O3 trondhjemitic gneisses occur in enclaves within granites at Markampara in the Bastar Craton, Central India. These gneisses exhibit a highly fractionated rare earth element (REE) pattern, [(frsol|La/Yb)n=60–83] and may have been produced by partial melting of an amphibolitic protolith. A zircon UPb age of 3509+14−7 Ma dates primary crystallization of the trondhjemites. A granite surrounding the gneissic enclaves yields a zircon UPb age of 2480 ±3Ma. RbSr wholerock data for the trondhjemitic gneisses are somewhat disturbed and indicate initial 87 Sr 86 Sr ratios of ∼0.7016. Pb isotopic data are poorly correlated and suggest a three-stage evolution with an early period of high UPb growth (μ≈9.5–10.5) after emplacement at 3.5 Ga, followed by isotopic disturbance and U depletion at 2.5 Ga during widespread magmatism and metamorphism. Loss of Rb and other mobile elements may also have occurred at this time.

Dating low-grade metamorphic events by SHRIMP U-Pb analysis of monazite in shales

Although the U-Pb isotopic system is widely used to date magmatism and medium- to high-grade metamorphism, dating low-grade metamorphic events has been hampered by a scarcity of suitable minerals. We show that monazite forms in shales during low-grade metamorphism and can be dated by in situ U-Pb analysis, providing precise ages for low- temperature thermal events. In Proterozoic shales from the Pine Creek inlier of northern Australia, monazite and xenotime crystals that formed in a contact aureole yield ages synchronous with granite intrusion, ca. 1830 Ma. In the Pilbara craton of northwestern Australia, monazite growth 2192 ± 5 m.y. ago coincided with basaltic volcanism and rifting, an event apparently responsible for hydrothermal iron-oxide mineralization in a nearby giant iron-ore deposit. The mineralizing environment was highly oxic and bathed by meteoric fluids; thus, our results imply the existence of an oxygen-rich atmosphere by 2200 Ma. Given the abundance of shales in the stratigraphic record, and the likely widespread distribution of monazite in such rocks, in situ U-Pb dating of monazite may become an important chronometer for low-grade metamorphic events.

Deducing the ancestry of terranes: SHRIMP evidence for South America–derived Gondwana fragments in central Europe

We present here an example of how the sensitive high-resolution ion microprobe (SHRIMP) zircon dating method can provide a terrane-specific geochronological fingerprint for a rock and thus help to reveal major tectonic boundaries within orogens. This method, applied to inherited zircons in a ca. 580 Ma metagranitoid rock from the eastern Bohemian Massif, has provided, for the first time in the central European Variscan basement, unequivocal evidence for Mesoproterozoic and late Paleoproterozoic geologic events ca. 1.2 Ga, 1.5 Ga, and 1.65–1.8 Ga. The recognition of such zircon ages has important consequences because it implies that parts of the Precambrian section of Variscan central Europe were originally derived from a Grenvillian cratonic province, as opposed to the common assumption of an African connection. A comparison with previously published SHRIMP data suggests, however, that these Mesoproterozoic and late Paleoproterozoic zircon ages may be restricted to the Moravo-Silesian unit in the eastern Variscides, whereas the Saxothuringian and Moldanubian zones appear to contain a typical north African (i.e., Neoproterozoic plus Eburnian) inherited-zircon age spectrum. This finding supports new tectonic concepts, according to which Variscan Europe is composed of a number of completely unrelated terranes with extremely different paleogeographic origins. The Moravo-Silesian unit can be best interpreted as a peri-Gondwana terrane, which was situated in the realm of the Amazonian cratonic province by the late Precambrian, comparable to the Avalonian terranes of North America and the United Kingdom.

Synmetamorphic lode-gold deposits in high-grade Archean settings

Lode-gold deposits that formed under conditions of the middle to upper amphibolite and granulite facies have been identified in the Yilgarn block of Western Australia. These deposits, which form a continuum with lower metamorphic grade (mesothermal) gold depos-its, show that at least some of the fluids responsible for mesothermal gold deposits originated in or below the middle crust. Studies of 18 O suggest that fluid compositions were controlled by the wall rocks (at least for some elements). These more proximal gold deposits provide more information about deep fluid sources than do investigations of higher-level mineralization deposited from fluids strongly modified by wall-rock interaction during the early part of their ascent.

Textural and geochemical discrimination between xenotime of different origin in the Archaean Witwatersrand Basin, South Africa

Abstract Xenotime (YPO4) of detrital, diagenetic, and hydrothermal origin within siliciclastic rocks of the Archaean Witwatersrand Basin, South Africa, has been identified on the basis of petrography and in situ ion microprobe (SHRIMP) age data. The chemical composition of xenotime, determined by in situ electron microprobe analysis, can be correlated with its origin. This allows the origin of any xenotime grain to be assessed by a non-destructive microanalytical method prior to ion microprobe geochronology. The main chemical discriminators are MREE-HREE abundance, normalised HREE slope and Eu anomaly, and, in some cases, U and Th contents. Igneous-detrital xenotime (> 2800 Ma) is distinguished from diagenetic (∼2780 Ma) and hydrothermal (