西藏措勤隆格尔铁矿岩体成岩时代及其地质意义

西藏措勤县的隆格尔铁矿位于拉萨地块隆格尔-工布江达弧背断隆带的西段,是冈底斯西段中生代矽卡岩型铁矿中重要的矿床之一。野外和室内工作表明,隆格尔铁矿床属矽卡岩型铁矿,与成矿有关的岩体为粗粒二长花岗岩。因此,本文通过对隆格尔铁矿成矿岩体的LA_ICP_MS和SHRIMP锆石U_Pb定年来探讨其成矿时代。分析结果表明:隆格尔粗粒二长花岗岩年龄为115.5±2.1 Ma,可近似代表铁成矿年龄。隆格尔铁矿与其东侧同处于同一构造单元内的尼雄铁矿床成岩成矿年龄为110~116 Ma,两者间的洛布勒铁矿床的成岩成矿年龄为111.3±1.6 Ma。这些铁矿床处于相同的构造单元,具相似的成矿特征和相近的成岩成矿年龄,构成一条东西向展布的早白垩世矽卡岩型铁矿成矿带,并可能向西延伸包括帮部勒、龙认拉、查加寺等矽卡岩型铁矿床。该带上还发育有晚白垩世的矽卡岩型铜金矿床,如日阿铜金矿床,成矿年龄均为87 Ma。因此,这一成矿带应当具有相同或相似的地质背景和构造-岩浆演化过程。隆格尔铁矿床乃至整个成矿带成铁的岩浆活动可能与洋壳断离前的板片回卷过程相关,成铜金的岩浆活动可能与洋壳断离过程相关,而矿区内石英闪长岩的侵入处于两者之间。

Petrogenetic epoch of the rock mass in the Lunggar iron deposit of Coqen County, Tibet

Lying in the western segment of Lunggar-Gongbo''gyamda arc back faulted-uplifted belt of the Lhasa terrane, the Lunggar iron deposit in Coqen County of Tibet is one of the important ore deposits in the Mesozoic skarn iron belt in the western part of Gandese. Field and indoor studies show that the Lunggar iron deposit is a skarn deposit related to coarse-grained monzogranite. In this study, LA-ICP-MS and SHRIMP zircon U-Pb dating of the metallogenic rock mass in the Lunggar iron deposit was conducted to probe into the ore-forming epoch of this deposit. The analytical result shows that the coarse-grained adamellite has an age of 115.5±2.1 Ma, which can approximately represent the ore-forming age of iron. The Lunggar iron deposit and the Nyicung iron deposit on the eastern side of the Lunggar iron deposit have rock-forming and ore-forming ages of 110~116 Ma, and the Luobule iron deposit between them have rock-forming and ore-forming ages of 111.3±1.6 Ma. These iron deposits are located in the same tectonic unit, have similar ore-forming characteristics and ore-forming as well as rock-forming ages, and constitute a EW-trending early Cretaceous skarn iron metallogenic belt which possibly extends westward and includes such skarn iron deposits as Bangbuqing, Longrenla and Chajiasi. In this belt, there are also late Cretaceous skarn copper-gold deposits such as the Ri''a copper-gold deposit whose ore-forming age is 87Ma. Therefore, this ore-forming belt should have the same or similar geological settings and tectonic-magmatic evolution processes. The iron-forming magmatic activity of the Lunggar iron deposit and even of the whole ore-forming belt might have to do with the underbow process of the plate, the copper- and gold-forming magmatic activity might have been related to the faulting and separating process of the oceanic crust, and the intrusion of the quartz diorite might have occurred between them.