喜马拉雅造山带晚中新世麻迦淡色花岗岩的构建机制

Assembling Mechanism for the Late-Miocene Mabja Leucogranite Pluton,Himalayan Orogen

在北喜马拉雅萨迦片麻岩穹窿西南侧发育有麻迦淡色花岗岩体,出露于南北向申扎—定结裂谷正断层的下盘,属一处较大规模的晚中新世淡色花岗岩体。该岩体具有较均一的元素和同位素(Sr和Nd)组成,但与多数喜马拉雅淡色花岗岩相比,具有异常高的(^(87)Sr/^(86)Sr)_i比值(0.85033~0.85034)和异常低的_(εNd)(t)值(-19.26^-18.30)组成,指示其部分熔融源区有更成熟古老地壳物质的参与。麻迦淡色花岗岩SHRIMP锆石U-Pb定年结果显示:1该岩体主要记录了至少两阶段岩浆结晶作用,分别发生在11.6±0.2 Ma和9.6±0.2 Ma;2个别13.8~16.0 Ma的岩浆作用年龄;3多数锆石继承核年龄分布于泛非期,少数年龄为中元古代(1558~1584 Ma)。在麻迦淡色花岗岩体南侧约40km处的日玛那穹窿,同位于申扎—定结南北向正断层的下盘,出露有大量原岩年龄为古元古代的日玛那糜棱岩,元素地球化学特征上类似于变泥质岩,显示高SiO_2(70.6%~74.6%),Al_2O_3(12.3%~14.0%),K_2O(4.22%~4.93%),A/CNK(1.50~1.58)和K_2O/Na_2O(1.42~2.18),代表了部分熔融源区可能存在的古老地壳物质岩石单元。因此,以麻迦淡色花岗岩为代表的北喜马拉雅晚中新世地壳深熔作用可能与青藏高原后碰撞阶段东西向伸展作用相关,泛非期变泥质岩及少量日玛那糜棱岩所代表的更古老岩石单元在16.0 Ma开始发生部分熔融,并在11.6 Ma至9.6 Ma之间达到深熔作用峰期,熔体活动可能持续了~2myr,以岩脉汇聚的形式延南北向正断层上升,构成侵位至北喜马拉雅特提斯沉积岩系之中的晚中新世麻迦淡色花岗岩体。

Mabja lagre leucogranitic pluton emplaced at the southwestern of North Himalaya Sakya dome during the Late Miocene period and located at the footwall of Xainza-Dinggye S-N normal fault. The 10Ma Mabja leucogranite have homogeneous elements and isotopic （Sr and Nd） compositions, however, obviously different from the great of Himalaya leucogranites, it is show relatively unradiogenic Sr （87Sr/86Sr（i）=0.85033～0.85034） and Nd （εNd（i）=-19.26～-18.30） isotope compositions, indicate that the Mabja leucogranite may be resulting from anatexis of much more mature crust materials. The SHRIMP zircon U-Pb dating results of the Mabja leucogranite show： （1）This pluton records at least two stage magmatic crystallization, happened at 11.6±0.2 Ma and 9.6±0.2 Ma respectively; （2） A few 13.8～16.0 Ma magmatic crystallized zircon ages; （3） most of the inherited zircon core ages are distributed to Pan-Africa period, whereas exists a few Paleoproterozoic ages （1558～1584 Ma）. 40 km ward the south, a suit of Paleoproterozoic mylonites distribute at the Ama Drime Massif, which also located at the footwall of Xainza-Dinggye normal fault. The elemental compositions of these mylonites are quite like meta-pelites of High Himalaya Crystalline Sequence, show high SiO2 （70.6%～74.6%）, Al2O3 （12.3%～14.0%）, K2O （4.22%～4.93%）, A/CNK （1.50～1.58） and K2O/Na2O （1.42～2.18）, represent the major anatexis mature crust materials that forming Mabja leogranitic pluton. Related to the E-W extension along the southern Tibetan rift system, a Late Miocene event of anatexis took place in North Himalaya region. The mixing source of meta-pelites and mature crust materials began to be melting at 16.0 Ma, reached its anatexis peak between 11.6 Ma to 9.6 Ma, and then drawing up along the S-N normal fault, emplaced in the Tethy Himalaya sequence and formed the Late Miocene leucogranitic plutons.