摘要
麻池河乡和沙河湾花岗岩体位于商丹断裂南侧。锆石的LA-ICP-MS U-Pb年代学分析表明,麻池河乡花岗岩岩浆结晶年龄为(490.8±2.9)Ma(加里东期早奥陶世);沙河湾石英二长岩岩浆结晶年龄分别为(240.6±1.5)^(228.2±1.5)Ma(印支期中三叠世)。麻池河乡花岗岩锆石两阶段Hf模式年龄(tDM2)为759~1 096Ma。沙河湾石英二长岩tDM2为891~1 516Ma。麻池河乡花岗岩源岩为新元古代和中元古代亏损地幔物质。沙河湾石英二长岩的源岩主要为中元古代壳幔混合物质,还含有少量新元古代壳幔物质。晚寒武世—早奥陶世秦岭洋壳向北俯冲于北秦岭地块之下,俯冲的洋壳和上覆地幔相互作用产生的杂化熔液通过结晶分异形成麻池河乡花岗岩。沙河湾岩体经历了至少两期地质事件,时间跨度为(240.6±1.5)^(228.2±1.5)Ma和214~197Ma。约250Ma勉略洋闭合之后,扬子板块和华北板块在秦岭地区发生碰撞,导致扬子陆块俯冲至南秦岭地块之下并发生小规模的部分熔融形成早—中三叠世((240.6±1.5)^(228.2±1.5)Ma)花岗岩类。约220Ma碰撞结束后,板片断离诱发软流圈物质上涌,同时俯冲陆壳开始折返,在地幔热和构造减压的条件下,俯冲陆壳及上覆岩石圈地幔发生广泛的部分熔融,形成不同程度具埃达克岩质地球化学特征的晚三叠世(214~197Ma)花岗岩类及伴生的镁铁质暗色包体。
Machihe country and Shahewan granitoid rock masses are located in the south of Shang Dan fault. Zircon laser ablation inductively coupled plasma mass spectrometry （LA-ICPMS） was used for U Pb chronologic analysis to determine crystallization ages for Machihe country granite and Shahewan quartz monzonite magma of 490. 8 ± 2. 9 Ma （Early Ordovician）, and （240. 6 ±1.5）-（228. 2±1.5） Ma （Middle Triassic）, respectively. The results of zircon in-situ Lu Hf isotopic analysis indicate that the two stage Hf model ages （t_DM2） of zircon are 759 - 1096 Ma and 891 - 1516 Ma, respectively. The source-rock material of Machihe country granite is Neoproterozoic and Mesoproterozoic depleted mantle material. Shahewan quartz monzonite is the mixing product of crustal with mantle materials at different levels in the Mesoproterozoic and Neoproterozoic time. In the northward subduction of the QinLing oceanic crust beneath North Qinling, the oceanic crust and the upper mantle interacted to produce the hybridized liquid and formed Machihe country granite by crystallization differentiation in the Irate Cambrian to the Early Ordovician. Shahewan rock mass was formed during at least two geological events with large time spans between （240. 6± 1.5）-（228.2 ± 1.5） Ma and 214 - 197 Ma, respectively. After the closure of the Mian-LiAe oceanic basin at about 250 Ma, a collision event occurred between the Yangtze and the North China plates in the Qinling orogenic belt. This collision resulted in the subduction of the continental Yangtze plate beneath South Qinling, in which a small part of the crust melted to form the Early-Middle Triassic granitoids at （ 240. 6 ± 1. 5）- （228. 2±1. 5） Ma. After this collision, approximately at 220 Ma, the detachment of the slab induced the upwelling of the asthenosphere material. The subducted continental crust began to uplift, and under the conditions of mantle heat and tectonic stress, widespread partial melting of the subducted continental crust and the upper lithosphere mantle occurred to form granitoids with various degrees of adakite characteristics in the Late Triassic （214- 197 Ma） and associated with mafic inclusions.
出处
《地学前缘》
EI
CAS
CSCD
北大核心
2013年第5期36-56,共21页
Earth Science Frontiers
基金
中国地质调查局项目(1212011120160
1212010611803
1212010711816)
国家专项(SinoProbe-05-05)
国家自然科学基金创新群体项目"青藏高原大陆动力学及资源环境效应"(40921001)
