Metamorphic evolution of the Sesia-Lanzo Zone, Western Alps: Time constraints from multi-system geochronology
Metamorphic evolution of the Sesia-Lanzo Zone, Western Alps: Time constraints from multi-system geochronology
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DOI：
10.1007/s004100050241
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发表时间：
1996-12-01
影响因子：
3.5
通讯作者：
Rex, DC
中科院分区：
文献类型：
--
作者：
Inger, S;Ramsbotham, W;Rex, DC
塞西亚-兰佐带是一个由海西期麻粒岩遗迹和高寒造山期榴辉岩、绿片岩相变质变形叠加而成的多变质单元。单元的不同部分记录了不同的p - t变形演化阶段，允许多系统同位素研究揭示变质历史的精确时间。新的Rb-Sr白云母和U-Pb榍石资料限制了榴辉岩相变质和变形的年龄为60-70 Ma。这大大改变了该单元早白垩世至中白垩世榴辉岩相变质作用的普遍观点。新结果与已建立的高山造山带大地构造格架更为一致，因为它们不需要在区域伸展时长时间的地温梯度下降。它也与最近对其他单位的研究更为一致，证明了白垩纪后高压变质作用。阶梯式加热的40Ar-39Ar分析得到了较好的平台，给出了比相应的Rb-Sr年龄更大的年龄。这种异常高的年龄表明，在高压变质事件的流体/压力条件下，晶界网络中存在放射性富氩流体。在多变质岩中，U-Pb榍石的年龄是可变的，在高压事件中显示出较老的Pb或榍石晶体的继承。两个单变质组合的年龄一致，为66 +/- 1 Ma，反映了榴辉岩相组合的结晶。片麻岩Minuti杂岩（GMC）在构造上位于榴辉云母岩之下，其普遍存在的绿片岩相组构形成紧密簇状的Rb-Sr白色云母，年龄在38 ~ 39 Ma之间。这次绿片岩事件并没有影响到大部分EMC。此时形成的云母的40Ar-39Ar年龄受到了很大的干扰，而从早期高压组合中幸存下来的云母的40Ar-39Ar系统被重置。绿片岩事件对海西期spenes的U-Pb系统影响不大，说明GMC在阿尔卑斯旋回期间并没有均匀地发生榴辉岩相变质作用，而是在造山后期与EMC并置。这个模型仍然要求变形和变质的轨迹（可能还有流体通量）随着时间的推移向外移动，使赛西亚-兰佐基底成为造山带内一个受剪切约束的无反应块体。
The Sesia-Lanzo Zone is a polymetamorphic unit containing Hercynian granulite relics overprinted by eclogite and greenschist facies metamorphism and deformation during the Alpine orogeny. Different parts of the unit record different stages on the P-T-deformation evolution, allowing multi-system isotopic studies to unravel the precise timing of the metamorphic history. New Rb-Sr white mica and U-Pb sphene data constrain the age of eclogite facies metamorphism and deformation to 60-70 Ma. This substantially alters the common view of early- to mid-Cretaceous eclogite facies metamorphism in this unit. The new results are more consistent with the established geotectonic framework for the Alpine orogeny, since they do not require a prolonged period of depressed geothermal gradient at a time when the region was in extension. It is also more concordant with recent studies of other units that demonstrate post-Cretaceous high-pressure metamorphism. Step-heated 40Ar-39Ar analysis of phengites yields good plateaux giving ages older than the corresponding Rb-Sr age. Such anomalously high ages indicate the presence of radiogenic argon-rich fluids in the grain boundary network under the fluid/pressure conditions acting during this high-pressure metamorphic event. The U-Pb sphene ages are variable in poly metamorphic rocks, and show inheritance of older Pb or sphene crystals into the high-pressure event. Two monometamorphic assemblages yield concordant ages at 66 +/- 1 Ma, reflecting crystallisation of the eclogite facies assemblage. The Gneiss Minuti Complex (GMC) lies structurally below the Eclogitic Micaschists, and its pervasive greenschist facies fabric yields tightly clustered Rb-Sr white mica ages at 38-39 Ma. This greenschist event did not affect the majority of the EMC. The 40Ar-39Ar ages of micas formed at this time were very disturbed, whereas micas surviving from an earlier higher pressure assemblage had their 40Ar-39Ar system reset. The greenschist event did not strongly affect U-Pb systematics in Hercynian age sphenes, suggesting that the GMC did not uniformly suffer an eclogite facies metamorphism during the Alpine cycle, but was juxtaposed against the EMC later in the orogeny. This model still requires that the locus of deformation and metamorphism (and possibly fluid flux) moved outboard with time, leaving the Sesia-Lanzo basement as a shear-bounded unreactive block within the orogenic wedge.