The Geological And Geochemical Features And The Genesis Of The Sazhai Granite In Yunnan Province

The Sazhai-dazhai granites of the Sanjiang region in Yunnan province, locatedbetween Changning-menglian tectonic melange belt and southern Lancangjiang faults,were regarded as Palaeoproterozoic metamorphic rocks or Mesoproterozoic granites.Based on a combination of field geological investigation on the Sazhai-dazhai granite,combined with a study of the lithological, chronological and geochemistrycharacteristics, this paper aims to determine their petrography, origine and tectonicsetting.Research has shown that the Sazhai-dazhai granite belongs to a suite ofMesozoic-Cenozoic granitoids rather than to Mesoproterozoic granites. The rocks aredominated by granites with a relict of metamorphic rocks. These granites were formedduring Indosinian and Himalayan.The main rock type of the Indosinian granite is monzogranite, with a majorelement composition that is high in SiO2and Al2O3and low in TiO2, P2O5and MgO.K2O and Na2O contents are moderate, and the A/CNK ration is greater than1.1,indicating that the monzogranite belongs to S-type granites that include theperaluminous and high-K calc-alkaline to peraluminous shoshonite series. The zirconU-Pb age for these granites is between204-221Ma (i.e., Indosinian). The ages of theinheritable zircons, however, demonstrate the involvement of Neoproterozoic crust and,as such, exclude the possibility of Mesoproterozoic crust in their genesis. The REEcompositions and patterns show clear enrichment in light REE over heavy REE. TheSazhai-dazhai granites can be classified into two types: one that is Rb, K and Pb,enriched, with negative Eu anomalies, but slightly Ba, Sr and Ti depleted, and with Hfshowing a weakly enriched character. The second group are characterized by positiveEu anomalies, and whose Rb, K and Pb are enriched, while Ba and Sr show weak to nodepletion and in terms of HFSE, have Ti showing strong depletion. On the basis of theRb-(Y+Yb) and the Nb-Y tectonic setting discrimination diagrams, the Sazhai-dazhaigranites can be classified as syn-collisional granitoids, with their genesis related to thecollisional orogeny of the Burma-Thai-Malaysia and Simao blocks, that occurredduring the Late Triassic. The dominant rock type of the Himalayan granite is monzogranite, as well; it hassimilar major elements characteristics to the Indosinian granitoids, both belong to theS-type granite, which is the peraluminous shoshonite series. The zircon U-Pb age forgranites is between20-38Ma, implying that its formation is related to the Himalyanorogeny, from the Oligocene to Miocene. The REE patterns for these rocks arecharacterized by enriched LREE and depleted HREE, with negative Eu anomalies.Trace elements analysis shows that Rb, K and Pb are enriched, while Ba and Sr arerelatively depleted; Hf is enriched, while Ti is relatively depletion. This granites wereascribed to the syncollisional-postcollisional granites in both Rb-(Y+Yb) and Nb-Ytectonic setting discrimination diagrams, having formed during the transitional stagefrom syncollisional to postcollisional period of the collision orogeny between theBaoshan block and Simao basin, that occurred during the Late Triassic.