Geochronology And Tectonic Significance Of Paleozoie Granites In The East Altyn-North Qaidam, Western China

The Paleozoie tectonic evolution of the East Altyn-North Qaidam in northeast Tibetan was very complex affected by Altyn Ocean, Paleo-Tethyan Ocean and North Qilian Ocean. The period of granitoids distribution were extremely widespread. This paper guided by plate tectonic is considering Geochronology, Geochemistry and Tectonic significance of Paleozoie Granites in the East Altyn- North Qaidam.The results indicate that:1)The concordia age of the hornblende biotite included granodiorite which was formed by the forth satge magmatism in the late Devonian is (368.7±3.5)Ma(MSWD=2.0).The geochemistry characters of the rock are initial 87Sr/86Sr 0.7126,0.7123, with initial 144Nd/143Nd 0.511838,0.511825, eNd -6.748044,-6.988785, and deplet mantle model age second stage 1460Ma,1480 Ma, initial 176 Hf /177Hf 0.282487, eHf(t) -2.157169, deplet mantle model age 1420.5 Ma.The zircon included granodiorite is deplet in LREE and rich in HREE,with Ce positive and Eu negative, SREE 5359.62-27201.68×10-6,average 11369.25×10-6, (Sm/La) N 1.24～720.74×10-6, average 121.6×10-6,indicate the source rock of it is the Proterozoic crust;2)The concordia age of the tonalite which was formed by the forth satge first times magmatism in the late Permian is (262.2±1.4)Ma(MSWD=1.2). The geochemistry characters of the roch are initial 87Sr/86Sr 0.7195, with initial 144Nd /143Nd 0.511861, eNd -8.559097, deplet mantle model age second stage 1510 Ma. The zircon included tonalite is also deplet LREE and rich HREE,with Ce positive and Eu negative, SREE 1944.57～29954.15×10-6, average 13850.43×10-6, (Sm/La) N 35.41～1414.34×10-6, average 504.53×10-6, (Lu/Gd) N 89.89～513.12×10-6, average 149.27×10-6; the concordia age of the quartz diorite is (261.8±1.5)Ma(MSWD=1.5), with initial 87Sr/86Sr 0.7059,0.7060, initial 144Nd/143Nd 0.512342、0.512344, eNd 0.772745,0.818588, and deplet mantle model age first stage 1224.9 Ma,1179.6 Ma, second stage 860 Ma. The zircon included quartz diorite is deplet LREE and rich HREE,with Ce positive and Eu negative, SREE 1967.11～29489.01×10-6, average 12713.98×10-6, (Sm/La) N 21.26～981.89×10-6, average 500.22×10-6, (Lu/Gd) N 67.44～452.32×10-6, average 164.28×10-6; the concordia age of the Huashigou granodiorite is (261.1±3.8)Ma(MSWD=3.2), following with initial 87Sr/86Sr 0.7070,0.7066, initial 144Nd/143Nd 0.512246,0.512266, eNd -1.238477 and -0.849065,and deplet mantle model age second stage 1000 Ma,970 Ma, initial 176Hf/177Hf 0.282766, eHf(t) 5.4554495,deplet mantle model age 876 Ma.That all can reveal the source rocks of this magmatism is the the Proterozoic crust;3)The concordia age of the monzogranite which was formed by the forth satge second times magmatism in the late Permian is (252.0±2.1)Ma(MSWD=1.7).The geochemistry characters of the roch are initial 87Sr/86Sr 0.7060,0.7061, with initial 144Nd/143Nd 0.512394,0.512404, eNd 1.692547,1.894787, and deplet mantle model age first stage 808.8 Ma,799.8 Ma, second stage 790 Ma,780 Ma, initial 176Hf/177Hf 0.282863, eHf(t) 8.7376777, deplet mantle model age 693.6 Ma, indicate the source rock of it is the Sinian crust;4)We argue that there were five periods of the granitic magmatism in the Paleozoic, (1) early to middle Ordovician (460～485 Ma), such as West Dapinggou and Saishiteng mountain rock;(2) late Ordovician (440～450 Ma), for example Kuoshibulake and Tataleng rock;(3) late Silurian to early Devonian (395～435Ma), for instance Luliangshan, Xitieshan and Yematan rock;(4) late Devonian (365～375Ma), such as Aolaohe and Huashigou hornblende biotite included granodiorite rock;(5) Permian (250～270Ma),Shanchagou rock and tonalite, quartz diorite, granodiorite and monzogranite of Huashigou.The four rocks of Huashigou is the result of two times magmatism,the first forming tonalite, quartz diorite, granodiorite,the second forming monzogranite.They were formed in different tectonic setting.