Geochronological evolution of HP metamorphic rocks of the Adula nappe, Central Alps, in pre-Alpine and Alpine subduction cycles

SHRIMP dating of zircon from eclogites, their country rock gneisses and quartz veins from the northern and middle Adula nappe, Central Alps, accompanied by cathodoluminescence imaging and REE geochemistry of these dated zircon crystals revealed that the Adula nappe has been affected by two pre-Alpine high-pressure (HP) metamorphic events (northern: <em>c</em>. 330–340 Ma; middle: <em>c</em>. 370 Ma). In the northern Adula nappe, loss of radiogenic Pb of the <em>c</em>. 330–340 Ma zircon domains in combination with previous geochronological data argue for an overprinting Alpine HP event. In the middle Adula nappe, metamorphic zircon domains record in addition a 33–32 Ma event, interpreted to reflect HT metamorphism, already identified in the southern part of the nappe, where such SHRIMP ages have been related to HT (Lepontine) metamorphism. The Alpine HP event indicated by previous SHRIMP data in the southern Adula nappe at <em>c</em>. 35 Ma is not recorded in the zircons of the rocks dated herein. The absence in this dataset is probably because either no HP (<em>c</em>. 35 Ma) zircon domains were formed or such domains were completely reset during the 33–32 Ma HT event. Protolith ages of orthogneisses and eclogites are 470–457 Ma and 595–587 Ma, respectively, and maximum sedimentation ages of paragneisses 500–460 Ma. The lack of Hercynian protolith zircon ages implies pre-Hercynian formation for the precursors of the dated HP rocks. Our data suggest that the Adula nappe contains various Hercynian basement slivers that underwent subduction to HP conditions at <em>c</em>. 330–340 Ma and <em>c</em>. 370 Ma prior to the Alpine orogeny, similar to many other parts of the Hercynian basement in Central and Southern Europe.

对中阿尔卑斯阿杜拉推覆体（Adula nappe）北部及中部的榴辉岩、围岩片麻岩及石英脉中的锆石开展敏感高分辨率离子探针（Sensitive High Resolution Ion MicroProbe，SHRIMP）定年，并对上述定年锆石开展阴极发光成像与稀土元素地球化学分析。结果显示，阿杜拉推覆体曾受到两期阿尔卑斯造山运动前的高压（HP）变质事件改造：北部推覆体的相关事件时限约为330~340 Ma，中部推覆体则约为370 Ma。在北部阿杜拉推覆体中，约330~340 Ma的锆石结构域出现放射成因铅丢失现象，结合前人年代学数据，可佐证存在一期叠加的阿尔卑斯期高压变质事件。中部阿杜拉推覆体的变质锆石结构域还记录了33~32 Ma的变质事件，该事件被解读为对应高温（HT）变质作用——该类高温变质作用此前已在该推覆体南部被识别，其对应的SHRIMP定年结果曾被关联至莱蓬蒂高温变质作用。此前基于SHRIMP数据提出的、南部阿杜拉推覆体约35 Ma的阿尔卑斯期高压变质事件，在本次研究定年的岩石锆石中并未被观测到。本次数据集未收录该事件信号，可能源于两方面原因：一是未形成约35 Ma的高压锆石结构域，二是该类结构域在33~32 Ma的高温变质事件中被完全重置。正片麻岩与榴辉岩的原岩年龄分别为470~457 Ma与595~587 Ma，副片麻岩的最大沉积年龄为500~460 Ma。本次研究未获得海西期原岩锆石年龄，指示本次定年的高压变质岩的前驱体形成于海西造山运动之前。我们的研究数据表明，阿杜拉推覆体包含多块海西期基底岩片，这些岩片在阿尔卑斯造山运动发生前，分别于约330~340 Ma与约370 Ma发生俯冲作用并达到高压变质条件，这一特征与中欧及南欧诸多其他海西期基底的地质演化模式一致。