Archaean calc-alkaline volcanism in the Pilbara Block, Western Australia
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The origin of basal lherzolites in the mantle section of harzburgite‐dominated ophiolites is enigmatic. The basal part of the mantle section is well exposed in the Muslim Bagh Ophiolite, Pakistan, which is one of the harzburgite‐dominated ophiolites of the Tethys Ophiolite Belt. In this contribution, we describe the basal lherzolite of the Muslim Bagh Ophiolite, Pakistan, and discuss its origin based on the trace‐element characteristics of its clinopyroxenes. The basal lherzolite exhibits porphyroclastic to mylonitic textures. Primitive mantle‐normalized trace‐element patterns of the porphyroclastic clinopyroxenes were characterized by low ratios of light rare‐earth elements (LREEs) to heavy rare‐earth elements (HREEs), low abundances of HREEs, and positive Sr anomalies. These geochemical characteristics are not consistent with a lherzolite and its clinopyroxenes, which is formed by a residue after a low degree of partial melting and melt extraction from peridotites in a mid‐ocean ridge setting. Instead, the compositions of the clinopyroxenes are consistent with open‐system melting induced by the infiltration of slab‐derived fluids into residual peridotites that had been depleted in REEs. The compositions of chromian spinels in the chromitites of the basal peridotite sequence are also consistent with their formation in an arc setting. We conclude that the basal lherzolites of the Muslim Bagh Ophiolite represent a residue after a relatively high degree of partial melting, and that the clinopyroxenes were added as metasomatic crystallization from slab‐derived arc‐related melts to this residual depleted peridotite in a subduction setting.
Peridotite
Metasomatism
Trace element
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The Archean crust contains direct geochemical information of the Earth's early planetary differentiation. A major outstanding question in the Earth sciences is whether the volume of continental crust today represents nearly all that formed over Earth's history or whether its rates of creation and destruction have been approximately balanced since the Archean. Analysis of neodymium isotopic data from the oldest remnants of Archean crust suggests that crustal recycling is important and that preserved continental crust comprises fragments of crust that escaped recycling. Furthermore, the data suggest that the isotopic evolution of Earth's mantle reflects progressive eradication of primordial heterogeneities related to early differentiation.
Hadean
Early Earth
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Several lines of evidence indicate that the Archean upper crust was considerably more mafic than the present-day upper crust. There has been no significant change in REE and Th abundances in post-Archean clastic sedimentary rocks, suggesting that there has been no change in the composition of the upper crust during the post-Archean. This indicates that if there have been any additions to the post-Archean upper crust, they must have had similar composition to the upper crust itself. Geochemical modelling of REE and Th abundances in sedimentary rocks suggests that the minimum ratio of post-Archean to Archean upper crustal composition required to eliminate the Archean upper crustal trace element signature, within analytical uncertainty, is about 4:1. Such a model also is supported by isotopic data. Using plausible assumptions regarding the volume of Archean crust, isostatic relations, and extreme models of the earth's degassing history, it is proposed that approximately 65-75% of the continental crust formed during the period of 3.2-2.5 Ga, and that between 70-85% of the continental crust had formed by 2.5 Ga. Such a model is consistent with relatively constant continental freeboard during the past 2,500 million years. Thus, the constant freeboard model does not provide unique evidence for large-scale recycling of continental material through the mantle.
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Several lines of evidence indicate that the Archean upper crust was considerably more mafic than the present-day upper crust. There has been no significant change in REE and Th abundances in post-Archean clastic sedimentary rocks, suggesting that there has been no change in the composition of the upper crust during the post-Archean. This indicates that if there have been any additions to the post-Archean upper crust, they must have had similar composition to the upper crust itself. Geochemical modelling of REE and Th abundances in sedimentary rocks suggests that the minimum ratio of post-Archean to Archean upper crustal composition required to eliminate the Archean upper crustal trace element signature, within analytical uncertainty, is about 4:1. Such a model also is supported by isotopic data. Using plausible assumptions regarding the volume of Archean crust, isostatic relations, and extreme models of the earth's degassing history, it is proposed that approximately 65-75% of the continental crust formed during the period of 3.2-2.5 Ga, and that between 70-85% of the continental crust had formed by 2.5 Ga. Such a model is consistent with relatively constant continental freeboard during the past 2,500 million years. Thus, the constant freeboard model does not provide unique evidence for large-scale recycling of continental material through the mantle.
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The Neo‐Tethyan ophiolites of the Tuting‐Tidding Suture Zone (TTSZ), Eastern Himalaya (viz., Tidding Ophiolite Complex and Mayodia Ophiolite Complex) and Indo‐Myanmar Orogenic Belt (IMOB, i.e., Nagaland Ophiolite Complex and Manipur Ophiolite Complex) which lie along the southern extension of the Indus‐Tsangpo Suture Zone have been collectively investigated through the mantle‐derived peridotite sequence. The peridotites (harzburgite and dunites) in the Eastern Himalaya ophiolites are refractory, constrained by the high Fo olivine content (~95), high Cr# [Cr/(Cr + Al)] (0.90–0.99), as well as by the parental melt composition of Cr‐spinels (Al 2 O 3 melt = 3.05–7.55 wt%, FeO/MgO = 0.69–6.46). They have slightly “U‐shaped” chondrite‐normalized Rare Earth Elements (REE) patterns with slight enrichment in Light Rare Earth Elements (LREE) relative to the patterns expected for residues of partial melting, thereby indicating a reaction with the LREE‐enriched melt. These peridotites represent residual portions of a depleted/enriched mantle that underwent partial melting up to 23% in the nascent forearc of an intra‐oceanic subduction zone, and later metasomatized by high‐temperature silicate melts and low‐temperature hydrous fluids. They are composed of a very refractory olivine‐spinel assemblage (Fo: 91.68–96.44; Cr#: 0.90–0.99), corroborating a boninitic parentage, with influence from melt‐rock interactions. In the case of the IMOB ophiolites, a wide range of chemical compositions is observed in the mantle sequence. Lherzolites display low Cr# (0.12–0.26) and TiO 2 (<0.11) associated with high Mg# [Mg/(Mg + Fet)] (0.69–0.76) in the Cr‐spinels present in them. They represent the residual product of a fertile mantle that underwent low‐degree partial melting (2%–10%) in a divergent mid‐ocean ridge (MOR) tectonic setting. On the other hand, the harzburgites and dunites of the IMOB have high Cr# (0.84–0.90) and low TiO 2 (<0.06 wt%) Cr‐spinels and exhibit slightly U‐shaped REE distributions indicating their derivation from a highly depleted mantle source, which experienced higher degree partial melting (17%–24%) similar to those of supra‐subduction zone (SSZ) harzburgites and dunites of the TTSZ peridotites. The occurrence of both MOR and SSZ types of melting regimes indicates that the peridotites in the IMOB ophiolites formed at two major different stages of the pre‐subduction and subduction events, respectively. However, the peridotites of Eastern Himalaya ophiolites were formed only during subduction tectonics. Thus, we argue that the mantle peridotites in the ophiolites of northeast India evolved from lherzolite through clinopyroxene‐harzburgite and harzburgite then to highly refractory dunite, supporting multistage melting and melt‐rock reaction processes during their generation.
Peridotite
Petrogenesis
Forearc
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