Early Triassic Gneissoid Granites in the Gaozhou Area (Yunkai Massif), South China: Implications for the Amalgamation of the Indochina and South China Blocks

摘要

The Indosinian granites are widespread in the South China block (SCB) and can give significant information regarding the early Mesozoic tectonic evolution of the SCB. A comprehensive geochronological and geochemical study is carried out on the Gaozhou gneissic granites from the Yunkai massif, in the southern part of the SCB. Laser ablation inductively coupled plasma mass spectrometry U-Pb dating of zircons indicates that the gneissic granites were formed at 246 +/- 3 Ma. These rocks are monzogranite in lithology and show features of medium-K calcalkaline series and weak peraluminous rocks. They have high SiO2 composition ranging from 75.85 to 76.84 wt, high alkalis contents with Na2O + K2O of 7.20-7.51 wt, and low MgO contents (0.11-0.20 wt). Rare earth element (REE) compositions of the granitic rocks are fractionated with enriched light REE and moderate Eu anomalies. The samples also show enrichment of large ion lithophile elements (Rb and K) and Pb and depletion of Sr, Ba, and high field strength elements (Nb, Ti, and P). They possess negative epsilon(Nd)(t) values of -7.1 to -4.8. Petrography and geochemical compositions of the gneissic granites indicate that they belong to I-type granite and are the result of partial melting of the early Paleozoic magmatic rocks in the region combined with minor assimilation of younger crustal components. We propose that the Indosinian gneissic granites in the Gaozhou complex might be the products of an early Indosinian collisional event at the southern part of the SCB. Integrating our new data with documented collision-related magmatic events during the Late Permian to Middle Triassic in the southern part of the SCB, we support the hypothesis of a collision between the Indochina block and the SCB during the Late Permian to Triassic period.