Zircon U–Pb geochronology and geochemistry of Devonian plagiogranites in the Kuerti area of southern Chinese Altay, northwest China: Petrogenesis and tectonic evolution of late Paleozoic ophiolites

Abstract

This study presents zircon U–Pb geochronology with major, trace, rare earth element and Sr–Nd isotope geochemistry for plagiogranite dyke swarms occurring within the gabbro unit of the lower part of the Kuerti ophiolite in southern Chinese Altay, Northwest China. These intrusive plagiogranites cut across the metamorphic zone consisting of amphibolites that are related to hydrothermal alteration and shearing of gabbros. LA‐ICP‐MS zircon U–Pb dating of 2 plagiogranite samples yields approximately 390 Ma age of origin. They are geochemically characterized by relatively high SiO2 and Na2O, low K2O, TiO2, and Al2O3 contents, with marked enrichment in light rare earth elements and depletion in Nb, Ta, and Ti. They have slightly higher initial 87Sr/86Sr ratios (0.7040 to 0.7049), lower εNd(t) values (+3.6 to +6.3), and higher Th contents (2.84–11.9 ppm) than those (0.7034 to 0.7048, +7.2 to +10.3, and 0.04 to 1.79) of closely associated amphibolites. Geological and geochemical attributes suggest that the plagiogranites were generated by the anatexis of hydrated and hydrothermally altered amphibolites and minor sediments from oceanic crust during shearing at low pressure (<0.1 GPa) and temperature (<850 °C) conditions in a suprasubduction zone setting. The ridge subduction at approximately 390 Ma caused the upwelling of the hot asthenosphere and triggered the spreading of the Kuerti back‐arc basin, followed by the formation of the plagiogranites in the Kuerti ophiolite.

An island arc origin of Jurassic plagiogranite in the Shiquanhe ophiolite, western Bangong Suture, Tibet: Zircon U–Pb chronology, geochemistry, and tectonic implications of Bangong Meso‐Tethys

“…The Shiquanhe plagiogranite samples are characterized with enriched LREEs, flat HREEs and negative Eu anomalies with Eu/Eu* ratios varying from 0.38 to 0.75 (Figure 6a). The primitive mantle normalization of trace elements of the plagiogranite samples exhibits selective depletion of large‐ion lithophile elements (LILEs) such as Ba and Sr and high‐field‐strength elements (HFSEs) such as Nb, Ta, and Ti, which is consistent with those of typical plagiogranites and different from those of continental granites (Figure 6b; Rollinson, 2009; Shen et al, 2017). The negative Eu and Sr anomalies may be related to the fractionation of plagioclase.…”

“…In the plot of K 2 O versus SiO 2 , all the samples can be grouped as low K 2 O series and fall into the oceanic plagiogranite field, which are different from that of continental granites (Figure 5c; Maniar & Piccoli, 1989; Yarmolyuk et al, 2016; Yang et al, 2017). Moreover, in the ternary plot the samples plot in the trondhjemite and tonalite fields and show less orthoclase character as typical oceanic plagiogranites, which distinguish the Shiquanhe plagiogranite from continental granites (Figure 5d; Coleman & Peterman, 1975; Shen et al, 2017; Xu et al, 2017; Wu et al, 2018).…”

“…(a) Chondrite‐normalized REE patterns and (b) primitive mantle‐normalized trace element spidergrams of the Shiquanhe plagiogranite. Data sources: fractionation‐type from Freund et al (2014); shear‐type from Shen et al (2017); subduction‐type from Li and Li (2003a); obduction‐type from Whitehead, Dunning, and Spray (2000); data for the Lagkorco plagiogranite are from Xu et al (2018); the chondrite and primitive mantle normalizing data are from Sun and McDonough (1989) [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…Plagiogranite in the study area occurs as lenticular bodies within the metabasite of the Shiquanhe ophiolite, and its length and width are almost 6 m and 5 m, respectively (Figure 2a,b). Metabasite surrounding the plagiogranite features stretching lineation and develops ocellar structure (Figure 2c,d), suggesting metamorphism due to shear deformation (Shen et al, 2017). In addition, a chilled margin is not observed between the plagiogranite and metabasite (Figure 2c).…”

“…Although seldom found and a silicic signature which differs from the ultramafic‐mafic rocks in oceanic crust, plagiogranite can provide crucial information for studying the origin and evolution of oceanic lithosphere (Grimes, Ushikubo, John, & Valley, 2011; Rollinson, 2014; Haase et al, 2016; Furnes & Dilek, 2017 and references there in). But the remarkable thing is that plagiogranite can be generated at different stages of oceanic crust evolution (Koepke, Berndt, Feig, & Holtz, 2007) and its genesis can be attributed to five models as follows: (1) extreme fractional crystallization of basaltic magma in oceanic crust (Freund, Haase, Keith, Beier, & Garbe‐Schönberg, 2014); (2) partial melting of shear metamorphosed mafic rocks in the shear zone of the lower oceanic crust third layer during the oceanic crust migration process (Pedersen & Malpas, 1984; Shen, Zhang, Wang, Saha, & Ma, 2017; Xu, Li, Fan, Wang, & Xie, 2018); (3) dehydration melting products of subducted oceanic crust under high pressure conditions (Yoshikawa & Ozawa, 2007); (4) anatexis of sedimentary rocks beneath ophiolite thrust fragments during the obduction of ophiolite onto the continental crust (Skjerlie, Pedersen, Wennberg, & De La Rosa, 2000); and (5) immiscibility products of felsic and iron‐rich basaltic magmas under anhydrous conditions (Dixon & Rutherford, 1979).…”