There is a strong genetic relationship between the petrogenesis of I-type granitoids and the evolution of continental crust in orogenic belts. This study of I-type granitoids in the East Kunlun orogen, Northern Tibetan Plateau, shows that reworking of old continental lithosphere is an important key to this genetic relationship. The East Kunlun has numerous Triassic granitic plutons that are related to subduction of the Palaeo-Tethyan ocean and terrane collision in the early Mesozoic. U–Pb analysis of zircons from these Triassic granitoids indicates that the granitic magmatism lasted from 249 to 223 Ma. Based on elemental and isotopic compositions and their petrogenesis, the magmatism can be divided into three groups. (1) Group 1 consists of quartz diorites and granodiorites (241–249 Ma), which are metaluminous high-K calc-alkaline I-type granitoids and exhibit typical subduction-related chemical characteristics. They were derived from lower crust mainly composed of Precambrian metabasaltic basement rocks with different degrees of involvement of mantle material. (2) Group 2 consists of granitic porphyries and syenogranites (231–238 Ma), which are high Rb/Sr, metaluminous to weakly peraluminous high-K alkali-calcic I-type granitoids, showing characteristics of typical pure crustal-derived granitoids. They were derived from partial melting of a Mesoproterozoic metagreywacke source in the lower crust. (3) Group 3 consists of porphyry granodiorites (c. 223 Ma), which are metaluminous high-K calc-alkaline I-type granitoids and exhibit the typical geochemical characteristics of adakites (e.g. high La/Yb and Sr/Y ratios and low Y and Yb contents). Their high K2O and low Mg# with evolved Sr–Nd–Hf isotopic compositions indicate that they were most probably derived from thickened mafic lower continental crust, which underwent partial melting induced by underplated hot mafic magma. Combining the present work with previous studies, we propose that the subduction of the Palaeo-Tethyan ocean lasted from 278 to 241 Ma, and the collision between the Bayan Har terrane and the East Kunlun occurred at 231–238 Ma, whereas the group 3 granitoids most probably formed in a post-collisional setting. Overall, all the studied I-type granitoids were derived from partial melting of old continental lower crust with minor addition of lithospheric mantle material; thus reworking of old continental lithosphere is an important mechanism for the evolution of orogenic crust.
Abstract Zircons and other heavy minerals (corundum, rutile, ilmenite, magnetite, sillimanite) are identified in the Nsanaragati gem corundum placer deposit, in the western part of the Mamfe sedimentary basin, SW Cameroon. These alluvial minerals have different morphological characteristics and zircons, in particular, vary mostly in colour and shape. They are reddish, brownish, yellowish, pink or colourless. These minerals form rounded and sub‐rounded alluvial grains, prismatic, pyramidal or dipyramidal crystals. Reddish zircons retain their original crystallographic shape. Trace element and U–Pb isotopic geochemical analyses of these reddish zircons, using the LA‐ICP‐MS method give significant Hf (4576–6334 ppm), Th (46–1565 ppm) and U (66–687 ppm) contents, with Th/U ratio ranging from 0.6 to 3.0. The 206 Pb/ 238 U corrected mean age gave 12.39 ± 0.55 Ma, which characterizes an Upper Cenozoic (Serravallian) magmatic event. The zircons are probably sourced from a magmatic field in the South eastern boundary of the Cross River Formation. The Cameroon Volcanic Line of basaltic and alkaline lavas and intrusions which lie east of the Mamfe Basin mostly range in age from 37 Ma to <1 Ma. The zircons may also relate to the Mount Bambouto plateau lavas which lie northeast of the Mamfe sedimentary basin and have an eruptive age range of 21–14 Ma. The oldest Nsanaragati reddish zircon ages overlap within error with the end stages of the Bambouto eruptions. This eruptive or a related episode provides a potential source for megacrystic reddish zircons within the Nsanagarati placer deposit.
Abstract Geochemical and petrological diversity within transcrustal magmatic systems usually reflects the magma properties and magmatic processes and thus is critical to understanding the origin of magmatic complexes and the evolution of continental crust. Herein, we present an integrated study on the petrology, mineralogy, geochronology, geochemistry, and Sr-Nd-Hf isotopes of Triassic mafic-felsic dikes in the East Kunlun orogenic belt, northern Tibetan Plateau, to elucidate the nature and evolution of the transcrustal magmatic system. The studied dikes intruding into the granodiorite pluton (ca. 235–233 Ma) comprise coeval ca. 220–218 Ma gabbroic diorite porphyry, diorite porphyry, granodiorite porphyry, and alkali-feldspar granite, resembling composite dike swarms. The macrocrysts in these dikes show various zoning patterns, indicating episodic magma recharge and crystal resorption. The compositional gap between the intermediate-mafic dikes (SiO2 = 52.9–67.8 wt%) and the granitic dikes (SiO2 > 75 wt%), as well as their homogeneous whole-rock Sr-Nd isotopes, with (87Sr/86Sr)i = 0.708387–0.710995 and εNd(t) = −5.83 to −4.34, but variable zircon Lu-Hf isotopes, i.e., εHf(t) = −7.67 to −0.36, demonstrates that magma mixing rather than cogenetic fractional crystallization accounts for their origin. In combination with thermobarometric insights, these results suggest that the mafic and felsic parental magmas originating from an enriched lithospheric mantle and ancient continental crust, respectively, were ultimately emplaced and stagnated at varying crustal depths (~22–30 km and 8–17 km). Subsequently, the felsic magma mush was replenished and rejuvenated by the underplated mafic magma, leading to varying degrees of crystal-melt and/or melt-melt mixing. This mush-facilitated crust-mantle magma mixing is an important mechanism accounting for the compositional diversity of the transcrustal magmatic system.
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