It is known that large amounts of Cenozoic high potassium volcanic rocks occur on the Tibetan plateau. The question is where do those high potassium magmas come from? Since the plateau is being compressed by subductio...It is known that large amounts of Cenozoic high potassium volcanic rocks occur on the Tibetan plateau. The question is where do those high potassium magmas come from? Since the plateau is being compressed by subduction and collision from the surrounding continents,it is a puzzle for us what is the formational environment and tectonic setting of these volcanic rocks? In particular,what is the relationship between these special rocks and formation and uplift of the plateau? We recently carried out an investigation on these Cenozoic volcanic rocks in Hoh Xil,northern Tibetan plateau,including volcanic rocks in the Jingyuhu,Xiongyingtai and Shuangquanzi areas. In the region,some older lava flows occur as thick sheets with a flat surface and weathering crust; while some relatively younger lavas remain a relict volcanic cone or vent. Most of the relict cones are small (<100m in diameter) and some are over hundred meters,distributed along faults,particular strike\|slip fault,e.g.,East Kunlun strike slip fault. Relatively,intermediate\|acid volcanic rocks are prominent,and basic\|intermediate and acid are subordinate. Petrological and geochemical results show that these volcanic rocks have many common affinities,characterized by high contents of w (K 2O)+ w (Na 2O) (about 8%),and w (K\-2O)/ w (Na\-2O)>1 or close to 1,and mostly belonged to shoshonite\|trachyte association and some to ultra\|potassic rocks. The K\|Ar dating of whole rock and mica shows that the volcanism lasts from middle Miocene (15 47Ma) to Pleistocene (0 69Ma); they are 0 69Ma,13 77Ma and 15 47Ma in Jingyuhu; 11 05Ma,12 22Ma and 12 83Ma in Xiongyingai,; 1 58Ma,2 24Ma,5 23Ma,5 85Ma,8 20Ma,8 41Ma and 10 67Ma in Shuangquanzi.Two types of xenolith and xenocryst were found in Jingyuhu and Xiongyingtai,i.e..,crustal xenolith and mantle xenocryst.展开更多
Cenozoic volcanism has been well studied in northern, eastern and southwestern Tibet (Coulon et al., 1986; Arnund et al., 1992; Turner et al., 1993, 1996; Deng, 1978, 1998; Miller et al., 1999, Wang et al, 2000). But ...Cenozoic volcanism has been well studied in northern, eastern and southwestern Tibet (Coulon et al., 1986; Arnund et al., 1992; Turner et al., 1993, 1996; Deng, 1978, 1998; Miller et al., 1999, Wang et al, 2000). But the data of the Cenozoic post\|collisional volcanism in central and southern Tibet is limited (Coulon et al., 1986; Turner et al., 1996; Zhang, 1998). These potassic and ultrapotassic intrusive and extrusive rocks are regarded to be a key clue for the deep lithospheric process of the plateau after Indo\|Asian collision. Present here is the preliminary results of the rocks from Oiyug (Wuyu) basin, about 150km northeast to Shigatse. Major and trace elements of the rocks are studied and compared with the ultrapotassic and potassic (shoshonitic) rocks exposed in Lhasa block and Qiangtang terrane.(1) Geologic setting. The magmatic rocks studied are Gazacun formation of the lower part of the Pliocene Oiyug group (N 2 oy ). The cross\|section is situated in Gazacun village of Namling. Gazacun formation is underlain unconformable by the andesitic rocks of Linzizong Group [(K 2—E 2) ln ], and covered by the coarse\|grain layered sandstone and conglomerate of Zongdangcun formation. The Gazacun formation consists of mediate\|acidic volcanics, granite\|porphyry, coal\|bearing clastics, plants and sporo\|pollen.展开更多
文摘It is known that large amounts of Cenozoic high potassium volcanic rocks occur on the Tibetan plateau. The question is where do those high potassium magmas come from? Since the plateau is being compressed by subduction and collision from the surrounding continents,it is a puzzle for us what is the formational environment and tectonic setting of these volcanic rocks? In particular,what is the relationship between these special rocks and formation and uplift of the plateau? We recently carried out an investigation on these Cenozoic volcanic rocks in Hoh Xil,northern Tibetan plateau,including volcanic rocks in the Jingyuhu,Xiongyingtai and Shuangquanzi areas. In the region,some older lava flows occur as thick sheets with a flat surface and weathering crust; while some relatively younger lavas remain a relict volcanic cone or vent. Most of the relict cones are small (<100m in diameter) and some are over hundred meters,distributed along faults,particular strike\|slip fault,e.g.,East Kunlun strike slip fault. Relatively,intermediate\|acid volcanic rocks are prominent,and basic\|intermediate and acid are subordinate. Petrological and geochemical results show that these volcanic rocks have many common affinities,characterized by high contents of w (K 2O)+ w (Na 2O) (about 8%),and w (K\-2O)/ w (Na\-2O)>1 or close to 1,and mostly belonged to shoshonite\|trachyte association and some to ultra\|potassic rocks. The K\|Ar dating of whole rock and mica shows that the volcanism lasts from middle Miocene (15 47Ma) to Pleistocene (0 69Ma); they are 0 69Ma,13 77Ma and 15 47Ma in Jingyuhu; 11 05Ma,12 22Ma and 12 83Ma in Xiongyingai,; 1 58Ma,2 24Ma,5 23Ma,5 85Ma,8 20Ma,8 41Ma and 10 67Ma in Shuangquanzi.Two types of xenolith and xenocryst were found in Jingyuhu and Xiongyingtai,i.e..,crustal xenolith and mantle xenocryst.
文摘Cenozoic volcanism has been well studied in northern, eastern and southwestern Tibet (Coulon et al., 1986; Arnund et al., 1992; Turner et al., 1993, 1996; Deng, 1978, 1998; Miller et al., 1999, Wang et al, 2000). But the data of the Cenozoic post\|collisional volcanism in central and southern Tibet is limited (Coulon et al., 1986; Turner et al., 1996; Zhang, 1998). These potassic and ultrapotassic intrusive and extrusive rocks are regarded to be a key clue for the deep lithospheric process of the plateau after Indo\|Asian collision. Present here is the preliminary results of the rocks from Oiyug (Wuyu) basin, about 150km northeast to Shigatse. Major and trace elements of the rocks are studied and compared with the ultrapotassic and potassic (shoshonitic) rocks exposed in Lhasa block and Qiangtang terrane.(1) Geologic setting. The magmatic rocks studied are Gazacun formation of the lower part of the Pliocene Oiyug group (N 2 oy ). The cross\|section is situated in Gazacun village of Namling. Gazacun formation is underlain unconformable by the andesitic rocks of Linzizong Group [(K 2—E 2) ln ], and covered by the coarse\|grain layered sandstone and conglomerate of Zongdangcun formation. The Gazacun formation consists of mediate\|acidic volcanics, granite\|porphyry, coal\|bearing clastics, plants and sporo\|pollen.
