Duration of strike-slip movements in large shear zones: The Red River belt, China
Duration of strike-slip movements in large shear zones: The Red River belt, China
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DOI：
10.1016/0012-821x(94)90119-8
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发表时间：
1994-09
影响因子：
5.3
通讯作者：
U. Schärer;Liangshang Zhang;P. Tapponnier
中科院分区：
文献类型：
--
作者：
U. Schärer;Liangshang Zhang;P. Tapponnier
用U--Pb法测定了红河剪切带中锆石、独居石、辉石和钛铁矿的年龄，对红河剪切带的左旋运动进行了时空约束。在15~20公里深处，剪切作用形成了高级变质片麻岩，它们构成了∼4200m高的点苍山和∼3100m高的哀牢山山脉的主体。片麻岩中无处不在的熔融层和岩袋表明，与构造变质作用有关的温度位于大陆地壳固相线及其上方，变形模式在整个带长范围内非常均匀。在片麻岩以北，次火山体沿着红河剪切带的投影走向侵入未变质的盖层，尤其是始新世红层。点仓山片麻岩中的淡花岗岩层，独居岩和辉石的年龄分别为2 4.2±0.2 Ma(2σ)和2 2.4±0.2 Ma，而锆石则在2 4~2 3 Ma之间经历了两个生长阶段。这些年龄记录了深熔熔体在至少1.8±0.4 Ma期间的固化，在此期间，左旋剪切持续活跃。哀牢山片麻岩中两个二长岩体的锆石和钛铁矿年龄分别为26.3±0.3 Ma和26.1±0.2 Ma，二长岩体内的两个伟晶岩层的年龄分别为24.1±0.2 Ma和22.4±0.2 Ma。这些结果表明，在熔融条件下(>；650℃)，锆石、独居石、钛铁矿可能也是铀和铅的封闭体系。因此，年龄是矿物在熔体中形成的日期，而不是在一定的封闭温度下冷却。点苍山片麻岩以北的二长岩次火山侵入岩年龄为35.0±0.1 Ma。由于这些侵入体遵循剪切带的走向，它们很可能代表了该带早期活动的表面表现，表明走滑运动在始新世末期已经发生。全套U-测年结果表明，岩浆和变质活动发生在35~20 Ma之间，这强化了红河剪切带吸收了32~16 Ma间南中国海开放所需的大部分板内活动的观点。红河带二长正长侵入岩的存在表明，熔融作用影响了点苍山和哀牢山片麻岩原岩以外的其他源岩。产生这种岩石的潜在来源位于地壳深处，可能包括来自地幔的熔岩。因此，红河剪切带一定是一个根深蒂固的结构，熔体沿着该结构运移到地表。此外，剪切运动和岩浆生成的同时性表明，熔融与走滑活动有直接关系。
Sixty four size fractions of zircon, monazite, xenotime and titanite were dated by the UPb method to temporally and spatially constrain left-lateral movements in the Red River shear zone. Between the depths of 15 and 20 km, the shearing led to the formation of the high-grade metamorphic gneisses that form the bulk of the ∼ 4200 m high Diancang Shan and ∼ 3100 m high Ailao Shan mountain ranges. The omnipresence of melt layers and pockets in these gneisses shows that temperatures associated with tectonometamorphism lie at and above the solidus of continental crust, and deformation patterns are very homogeneous over the entire length of the belt. North of the gneisses, subvolcanic bodies intrude unmetamorphosed cover rocks—in particular Eocene red beds—along the projected trend of the Red River shear zone.For a leucogranitic layer in the Diancang Shan gneisses, an age of 24.2 ± 0.2 Ma (2σ) is obtained for monazite and 22.4 ± 0.2 Ma is obtained for xenotime, whereas zircon formed through two growth stages at 24–23 Ma. These ages record solidification of the anatectic melts over a period of at least 1.8 ± 0.4 Ma, during which left-lateral shear was continuously active. In the Ailao Shan gneisses, zircon and titanite ages are 26.3 ± 0.3 and 26.1 ± 0.2 Ma for two monzonitic bodies, and 24.1 ± 0.2 and 22.4 ± 0.2 Ma for two pegmatitic layers that occur within the monzonitic intrusions. These results show that zircon, monazite, titanite and probably also xenotime are closed systems for U and Pb under melting conditions ( > 650°C). Therefore, the ages date mineral formation in the melt, rather than cooling below certain closure temperatures. A significantly older age of 35.0 ± 0.1 Ma is obtained for a monzonitic subvolcanic intrusion north of the Diancang Shan gneisses. Because these intrusions follow the trend of the shear zone, they most likely represent the surface expression of early activity along that zone, indicating that strike-slip movement had already occurred in latest Eocene time. The total set of UPb ages shows that magmatic and metamorphic activity occurred from 35 to 20 Ma, strengthening the idea that the Red River shear zone absorbed most of the intraplate movements required to compensate the opening of the South China Sea between 32 and 16 Ma. The existence of monzonitic-syenitic intrusions along the Red River zone implies that melting affected source rocks other than the Diancang Shan and Ailao Shan gneiss protoliths. Potential sources to produce such rocks lie in the deep crust, possibly including melts from the mantle. Consequently, the Red River shear zone must be a deep-rooted structure, along which melts migrated to the surface. Moreover, the contemporaneity of shear motion and magma generation suggests that melting occurred in direct relation to strike-slip activity.