Petrogenesis and Tectonic Implications of the Triassic Granitoids in the Ela Mountain Area of the East Kunlun Orogenic Belt

Abstract

The East Kunlun Orogenic Belt is located in the western part of the Central Orogenic Belt of China, with a large number of Triassic igneous rocks parallel to the Paleo-Tethys ophiolite belt, which provides a large amount of geological information for the tectonic evolution of the Paleo-Tethys Ocean. The granitoids studied in this paper are located in the Ela Mountain area in the eastern part of the East Kunlun Orogenic Belt. Zircon U-Pb dating results show that these different types of granitoids were crystallized in the Triassic. The 247.5 Ma porphyritic granites from Zairiri (ZRR) displayed calc-alkaline I-type granite affinities, with the zircon εHf(t) values being mainly positive (−0.5 to + 3.8, TDM2 of 1309–1031 Ma), indicating that they are derived from the partial melting of the juvenile crust and mixed with ancient crustal components. The 236.8 Ma Henqionggou (HQG) granodiorites and 237.5 Ma Daheba (DHB) granodiorites are high-K calc-alkaline I-type granite, and both have mafic microgranular enclaves (MMEs), showing higher and more varied Mg# (39.73–62.73), combined with their negative Hf isotopes (εHf(t) = −2.6 to −1.6, TDM2 = 1430–1369 Ma), suggesting that their primary magmas were the products of partial melting of the Mesoproterozoic lower crust that mixed with mantle-derived rocks. The 236.4 Ma DHB porphyritic diorites showed characteristics of high-K calc-alkaline I-type granitoids, with moderate SiO2 contents, medium Mg# values (40.41–40.65), with the Hf isotopes (εHf(t) = −2.9 to −0.5; TDM2 = 1451–1298 Ma) indistinguishably relative to contemporaneous host granodiorites and MMEs. The petrographic and geochemical characteristics indicate that the porphyritic diorites are the product of well-mixed magma derived from the Mesoproterozoic lower crust and lithospheric mantle. Based on the results of this paper and previous data, the chronology framework of Late Permian–Triassic magmatic rocks in the eastern part of the East Kunlun Orogenic Belt was constructed, and the magmatic activities in this area were divided into three peak periods, with each peak representing an extensional event in a particular tectonic setting, for example, P1 (slab roll-back in subduction period; 254–246 Ma), P2 (slab break-off in transition period of subduction and collision; 244–232 Ma), P3 (delamination after collision; 230–218 Ma).

Petrogenesis, Sources, and Tectonic Settings of Triassic Volcanic Rocks in the Ela Mountain Area of the East Kunlun Orogen: Insights from Geochronology, Geochemistry and Hf Isotopic Compositions

“…The intrusive and volcanic rocks of the P2 stage are widespread in the East Kunlun Orogen, but there is a great controversy about their formation environment, including post-collision [12,13], syn-collision [14][15][16], and subduction [17][18][19]. The volcanic rocks studied in this paper are in the range of the P2 stage; in the rock association of this stage, the intrusive rocks are mainly granodiorite [14,17,44], monzogranite, syenogranite [45,47], and a small amount of mafic-intermediate rocks [14,44,45]. The granodiorites mostly contain mafic microgranular enclaves [12,14,17,44], which have typical crust-mantle mixing characteristics, suggesting intensive crust-mantle interactions during this period.…”

“…The volcanic rocks studied in this paper are in the range of the P2 stage; in the rock association of this stage, the intrusive rocks are mainly granodiorite [14,17,44], monzogranite, syenogranite [45,47], and a small amount of mafic-intermediate rocks [14,44,45]. The granodiorites mostly contain mafic microgranular enclaves [12,14,17,44], which have typical crust-mantle mixing characteristics, suggesting intensive crust-mantle interactions during this period. Volcanic rocks are mostly rhyolite, dacite, a small amount of andesite and basaltic andesite, etc.…”

“…The chondrite values and primitive mantle values were from [43]. Previous data are from [3,14,16,18,44,45], and they represent igneous rock samples in the East Kunlun Orogen of the same period as the volcanic rocks studied in this paper.…”

“…Subsequently, the 239.3-234.4 Ma volcanic eruption and the simultaneous intrusive rocks were formed in the slab failure environment, representing the beginning of the initial extension of the late stage of syn-collision. [3,14,18,44,45], and they represent igneous rock samples in the East Kunlun Orogen of the same period as the volcanic rocks studied in this paper.…”

“…(a) La/Yb vs. Nb + Y diagram; (b) Nb/Y vs. Nb + Y diagram; (c) La/Yb vs. Ta + Yb diagram; (d) Nb/Y vs. Ta + Yb diagram; (e) Nb vs. Y diagram; (f) Rb vs. Y diagram. The previous data are from[3,14,18,44,45], and they represent igneous rock samples in the East Kunlun Orogen of the same period as the volcanic rocks studied in this paper.…”