Discovery Of Indosinian Amphibolite Schists In Yunnan Malipo Area And Its Geological Significance

Yunnan Malipo area is located in the southwest of South China plate, on the Southeast Yunnan of Huanan fold belt system,within the far northern end of the Yuebei ancient land which are dominated with basement metamorphic rocks. Some amphibolite piece veins were sporadically developed with in the vault, primarily in terms of bedded and lenticular formation. The study of amphibolite schist has important implications for tectonic evolution of this region, however insufficient research on their petrogenesis, formation age and tectonic significance were accomplished previously. In this paper, systematically mineral identification, geochemical analysis and zircon U-Pb geochronology tests were carried out on these amphibolites schist collected form the study area and its geological significance were further discussed.Petrography research indicates the amphibolite is primarily composed of plagioclase and hornblende and are expressed as alignment, and formed sheet management, namely D1S1 penetrative foliation. Its major occurrence were consistent with the occurrence of the gneissosity within the dome therefore are preliminary believed that its formation were closely related to the formation of metamorphic core complex dome.Geochemical analysis indicated the amphibolite schist composed of 48.59%-55.16% Si O2 with limited variance, has high Al2O3, Ti O2 and K20 content, total iron content of 7.75%-15.12%, total bases content of 3.04%-4.68%. The samples were inrich in light rare earth elements(LREE) and depleted of heavy rare earth elements(HREE), suggested the amphibolite schist experienced REE differentiation during metamorphism. Recovery results presumed the original rock of metamorphosed amphibolite schist was sub-alkali basalts. Tectonic setting discriminant analysis indicated that the original rock was intraplate basalt which formed within a continental rift tectonic setting.Dating results reveal that the metamorphic age of amphibole schist is 220±1Ma. Combing with the petrogenetic and mineralogical charactersitics, the metamorphic process for amphibole generated earlier than or identical to the D1S1 foliation, indicating that the heating episode for generating amphibole is not later than the formation of metamorphic complex. In addition, the D1S1 foliation folded in its later stage which is contemporaneously with the time that gneissic granite hosted in the dome folded, the D2S2 formed by then. The above information suggests that amphibole schist formed earlier than D2S2 foliation, and this metamorphic episode is earlier than the starting time of the thrust cover.Combined with field observation and discussion above, we conclude that the plagio amphibolite shist should be formed inside the cove in veins in the early stage. The plagio amphibolite shist then lifted to the surface in the form of metamorphic core complex together with the cove during the indosinian. Together they developed foliated gneissic schistosity regional metamorphism, forming D1S1. This age should be no later than 220 Ma. During the later over-thrust, as a part of the metamorphic core complex, the plagio amphibolite shist developed compressional structure such as drape and obduction zone with other rocks.