Appendix A: LA-ICP-MS results (Laser-ablation inductively coupled plasma mass spectrometer) from monazite Moacir standard

Three samples of garnet-kyanite paragneiss from the Variscan Ulten Zone (Northern Italy) were studied in detail for U-Th-Pb monazite dating. Monazite in these gneisses is abundant, shows highly variable grain size and occupies different textural positions: within the matrix, as inclusion in garnet and kyanite, within apatite aggregates. Monazite shows different deformation features as a function of the textural position: enclosed (shielded) monazite is generally more fractured than matrix (unshielded) monazite. The integration of textural information with deformation features and in situ U-Th-Pb analyses by LA-ICP-MS indicates that there is no direct correlation between textural site and monazite ages. Old ages of 351-343 Ma, determined on portions of large matrix (unshielded) monazite and on rare domains of monazite shielded by garnet, have been related to a prograde stage of the Variscan metamorphic evolution of the Ulten Zone. Ages of 330-326 Ma, which are related to the thermal peak, are recorded by small matrix monazite, external domains of large matrix monazite, and by (domains of) fractured monazite enclosed in garnet and kyanite. Large, old unshielded grains formed as blasts during the prograde metamorphic history and survived the peak metamorphism during which crystallisation/re-crystallisation partially occurred.

本次研究针对意大利北部华力西期乌尔特恩带（Variscan Ulten Zone）产出的3件石榴子石-蓝晶石副片麻岩（garnet-kyanite paragneiss）样品，开展了详细的独居石（monazite）U-Th-Pb定年分析。该类片麻岩中独居石含量丰富，粒径变化范围极大，且赋存于不同的结构位置：基质内部、石榴子石与蓝晶石的包裹体中以及磷灰石集合体内部。根据赋存结构位置的不同，独居石展现出各异的变形特征：被包裹（受屏蔽）的独居石通常较基质中未受屏蔽的独居石破裂程度更高。结合结构赋存信息、变形特征与激光剥蚀电感耦合等离子体质谱法（LA-ICP-MS）原位U-Th-Pb分析结果可知，结构位置与独居石定年结果之间并无直接关联。对大型基质未受屏蔽独居石的部分区域以及罕见的被石榴子石屏蔽的独居石区域测得的351~343 Ma的古老年龄，对应乌尔特恩带华力西期变质演化的进变质阶段。330~326 Ma的年龄数据对应变质热峰阶段，其记录载体包括小型基质独居石、大型基质独居石的外围区域，以及赋存于石榴子石和蓝晶石内部的破裂独居石区域。大型古老未受屏蔽的独居石晶粒形成于进变质演化历史中的变斑晶生长过程，并在峰期变质作用中得以保存，期间发生了部分结晶/再结晶作用。