桐柏山地区低温榴辉岩变质作用的P-T轨迹

P-T path of low-temperature eclogites from the Tongbaishan area.

桐柏山地区发育南、北两条低温高压榴辉岩带,中间被桐柏山高级变质杂岩所分隔。本文对两条带内的4个保存完好的榴辉岩体及其退变产物进行了详细的岩石学研究和 P-T 条件估算。结果表明,北带榴辉岩的形成温压条件较高,为530～610℃、1.7～2.0GPa,南带榴辉岩较低,为470～520℃、1.3～1.7GPa,说明两条榴辉岩带不属于同一个高压岩片。实际上,二者分别对应于西大别地区的浒湾榴辉岩带(I 带)和七角山榴辉岩带(V 带)。电子探针分析发现,所有榴辉岩中的石榴石都展示了进变质环带,而退变榴辉岩的成分环带更为复杂,即保留了早期的进变环带,也显示了晚期的低温取代以及后期相对高温条件下的再生长,由此为榴辉宕的变质作用推导出一个“发卡式”演化轨迹。这种轨迹表明,榴辉岩先是在降温条件下缓慢向上回返的,这与目前已建立的多数高压、超高压岩石的快速抬升模式有所差异,而后在其进入到中地壳后又在相对较高的地温梯度下遭受退变质作用的改造。

Two low-temperature and high-pressure eclogite zones, which are separated by the Tongbaishan high-grade complex, were recognized in the Tongbaishan area. The detailed petrological observation and P-T estimation on four eclogites and their retrograde equivalents indicate that the two zones were formed at different conditions, 530 ～610℃ and 1.7 -2.0GPa for the northern zone, and 470 ～520℃ and 1.3～1.7GPa for the southern zone. This suggests that the two zones do not belong to the same high-pressure slice, and correspond to the Huwan eclogite zone （ Zone Ⅰ） and the Qijiaoshan eclogite zone （ Zone Ⅴ） from western Dabieshan, respectively. Electron microprobe analyses reveal that all the garnets from eclogites show progressive chemical zoning. By contrast, the zonation of garnets from retrograde eclogites is more complicated. For a single garnet porphyroblast, an earlier prograssive recrystallization at increasing temperature is recorded in the core, a further replacement at lower temperature in the inner rim, and a later growth at higher temperature in the outer rim. If so, a “hairpin-type” P-T path could be deduced from the Tongbaishan eclogites. Such a path implies that the eclogites were slowly uplifted at decreasing temperature from the deep levels, which is different from the fast exhumation model established for the most high- and ultrahigh-pressure rocks. The later retrograde metamorphism of eclogites occurs at higher geothermal gradient in the medium crust.