Geochemical differences between subduction- and collision-related copper-bearing porphyries and implications for metallogenesis
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Abstract:
Porphyry Cu (–Mo–Au) deposits occur not only in continental margin–arc settings (subduction-related porphyry Cu deposits, such as those along the eastern Pacific Rim (EPRIM)), but also in continent–continent collisional orogenic belts (collision-related porphyry Cu deposits, such as those in southern Tibet). These Cu-mineralized porphyries, which develop in contrasting tectonic settings, are characterized by some different trace element (e.g., Th, and Y) concentrations and their ratios (e.g., Sr/Y, and La/Yb), suggesting that their source magmas probably developed by different processes. Subduction-related porphyry Cu mineralization on the EPRIM is associated with intermediate to felsic calc-alkaline magmas derived from primitive basaltic magmas that pooled beneath the lower crust and underwent melting, assimilation, storage, and homogenization (MASH), whereas K-enriched collision-related porphyry Cu mineralization was associated with underplating of subduction-modified basaltic materials beneath the lower crust (with subsequent transformation into amphibolites and eclogite amphibolites), and resulted from partial melting of the newly formed thickened lower crust. These different processes led to the collision-related porphyry Cu deposits associated with adakitic magmas enriched by the addition of melts, and the subduction-related porphyry Cu deposits associated with magmas comprising all compositions between normal arc rocks and adakitic rocks, all of which were associated with fluid-dominated enrichment process. In subduction-related Cu porphyry magmas, the oxidation state (fO2), the concentrations of chalcophile metals, and other volatiles (e.g., S and Cl), and the abundance of water were directly controlled by the composition of the primary arc basaltic magma. In contrast, the high Cu concentrations and fO2 values of collision-related Cu porphyry magmas were indirectly derived from subduction modified magmas, and the large amount of water and other volatiles in these magmas were controlled in part by partial melting of amphibolite derived from arc basalts that were underplated beneath the lower crust, and in part by the contribution from the rising potassic and ultrapotassic magmas. Both subduction- and collision-related porphyries are enriched in potassium, and were associated with crustal thickening. Their high K2O contents were primarily as a result of the inheritance of enriched mantle components and/or mixing with contemporaneous ultrapotassic magmas.Keywords:
Underplating
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Quartz monzonite
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The similarity and difference between C -type adakitic rocks in eastern China and typical Adakite in the Pacific Ocean in respect to geochemistry and tectonic setting are discussed.Mesozoic inter-mediate-acid igneous rocks with adakitic signatures in eastern China are considered as the product of in-tracontinental magmatism.It is inferred that they originated by partial melting of basic rocks in the lower crust due to underplating of basaltic magma from the lithospheric mantle to the bottom of the thickened continental crust(50km)with cratonic affinity.The geodynamic implications of Adakite,especially C-type Adakite,are dealt with in the paper.Adakite can be considered as a new series of island-arc magma-tism and its discovery marks a major breakthrough in the petrological study and initiates a new way for the study of granitoids rocks.Several possible tectonic settings for the formation of Adakite are also dis-cussed.Adakite is a key to the study of crustal thickening and relevant events in geological history and the backward deduction of the composition of the lower crust.In addition,the relation of Adakite to mineral-ization is also discussed.
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According to the thermal structure of the lithosphere, the continental crust is located within the range of the MBL (mechanical boundary layer) and its heat transfer is controlled by thermal conductivity of the rocks in the crust. Therefore, based on the theory of thermal conductivity and the same thermophysical parameters as those obtained in previous studies, we calculated the thermal state of the crust underplated by basaltic magma. The result suggests that a felsic magma layer, thinner than 250 m, would be generated at 850℃ by the emplacement of a basalt magma sill (1200℃, 500 m thick) into continental crust during a very short period (≤ 2700 a). Geological and geochemical characteristics and isotopic chronological data of the bimodal volcanic rock association in the Changpu Formation of the Yutian Group in the Longnan-Xunwu area of Southern Jianxi suggest that the rhyolite of this association might be derived directly from partial melting of the upper crust caused by emplacement of quartz tholeiite magma. The Mesozoic continental crust in SE China (≤50 km in thickness) was located in the MBL and the amount and formation age of the felsic magma generated by the underplating of basaltic magma would be constrained by heat conductivity of the rocks in the crust. Granite and rhyolitic rocks dominate (90%) the Mesozoic igneous rock in SE China, while basaltic rocks are minor with their formation ages different from those of felsic rocks. Therefore, the formation of the extensive felsic igneous rocks in SE China may have not been caused by the underplating of basalt.
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Intermediate to felsic volcanic rocks of the Baiqi formation from the Lingqiu basin in the central part of North China Craton were studied. Single zircon U-Pb dating indicates that these volcanics formed at 125.8±3.0 Ma. Their Sr and Nd isotopic compositions (143Nd/144Nd = 0.51180-0.51182, 87Sr/86Sr = 0.7062-0.7063) fall in the range of the nearby late-Mesozoic basaltic rocks. These volcanics share geochemical affinities to the adakites formed in the modern arcs, e.g., high Na2O (>4.06%), Al2O3 (>15.4%) and Sr (645-1389 ppm) contents and Sr/Y ratios (55∼103), and thus being termed as adakitic rocks. However, the Baiqi adakitic rocks were not temporospatially associated with active subduction. Furthermore, their low Cr (2.19-47.4 ppm, with average of 25) and Ni (1.57-20.7 ppm, with average of 12) contents and Mg# (22-47, with average of 32) argue against interaction with the lithospheric mantle. Combined with the geological setting, we suggest that the Baiqi adakitic rocks resulted from partial melting of a thickened lower continental crust associated two episodes of basaltic underplating events. We propose that enormous conductive heating from 80-140 Ma basaltic underplating resulted in partial melting of pre-existing mafic lower crust formed by ∼150-160 Ma basaltic underplating. This study provides a case for partial melting of the thickened lower continental crust in association with basaltic underplating events.
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