Geochronology and geochemistry of the Late Triassic Longtan pluton in South China: termination of the crustal melting and Indosinian orogenesis

• Published in: International journal of earth sciences : Geologische Rundschau Vol. 103; no. 3; pp. 649 - 666
• Main Authors: Qiu, Liang, Yan, Dan-Ping, Zhou, Mei-Fu, Arndt, Nicholas T., Tang, Shuang-Li, Qi, Liang
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.04.2014; Springer Nature B.V
• Subjects: Earth and Environmental Science; Earth Sciences; Geochemistry; Geology; Geophysics/Geodesy; Magma; Marine; Mineral Resources; Mineralization; Original Paper; Petrology; Sedimentology; Structural Geology; China; ISEW, Vietnam; China, People's Rep; Indosinian orogeny; High-K granodiorite and granite; Partial melting; South China; Fractional crystallization; Granitoid genesis
• ISSN: 1437-3254; 1437-3262
• DOI: 10.1007/s00531-013-0996-z

The Indosinian orogeny is recorded by Triassic angular unconformities in Vietnam and South China and by widely occurring granitoids in the Yunkai-Nanling and the Xuefengshan belts of South China. The Longtan pluton in the northwestern part of the Xuefengshan belt is a typical high-K, calc-alkaline, I-type granitoid, which can shed light on the relationship between the Indosinian tectonic and magmatic activity in the region. Three precise zircon U–Pb ages yielded a mean of 218 ± 0.8 Ma, which is taken as the age of crystallization. The pluton consists of both granodiorite (64.59–68.01 % SiO 2 and 3.25–4.22 % K 2 O) and granite (70.49–71.80 % SiO 2 and 4.07–4.70 % K 2 O). The granodiorites are characterized by relatively high Mg# (54–57), low contents of Na 2 O (3.2–4.3 wt%), low abundances of incompatible elements (LILE, Nb and P), high initial 87 Sr/ 86 Sr (0.7175–0.7184) and negative ε Nd ( t ) (−9.98 to −9.72). REE patterns show moderate fractionation ((La/Yb) cn  = 8.07–18.80) with negative Eu anomalies (Eu/Eu* = 0.62–0.86). Compared with the granodiorite, the granite has a wider range of Mg# (49–59), lower contents of Na 2 O (2.8–4.2 wt%), higher initial 87 Sr/ 86 Sr (0.7232–0.7243) and more negative ε Nd ( t ) (−12.07 to −11.24) values. REE patterns are relatively flat ((La/Yb) cn  = 14.73–29.37) with smaller negative Eu anomalies (Eu/Eu* = 0.48–0.63). The granodiorite has lower K 2 O/Na 2 O and Al 2 O 3 /(MgO + FeO Tot ) values than the granite. Based on major and trace element geochemistry and Sr–Nd isotopes, we interpret the Longtan granodioritic magma to have been derived by partial melting of interlayered Proterozoic metabasaltic to metatonalitic source rocks, whereas the granite was probably derived from a mixture of Proterozoic metagraywackes and metaigneous rocks. Field, petrographic and geochemical evidence indicate that partial melting and fractional crystallization were the dominant mechanism in the evolution of the pluton. The Longtan granodiorites and granites are petrologically and geochemically similar to typical Indosinian varieties and are considered to have been produced in a similar manner. The Indosinian granitoids in the region show a magmatic peak age of ~238 Ma from the Yunkai-Nanling belt in the southeast and a magmatic peak age of ~218 Ma of the Xuefengshan belt to the northwest. These early and late magmatic episodes of the Indosinian granitoids also display slight variations of regular compositions, ε Nd ( t ) values and T DM ages. Thus, we propose a syncollisional extension model that Indosinian granitoids were generated by decompressional partial melting of crustal materials triggered by two extensions during collision of the Indochina and South China blocks. The Longtan pluton in the northwesternmost part of the orogenic belt marks the termination of the Indosinian magmatism and orogenesis.