Petromorphology of the Shiren Mountain block in the Qinling orogenic belt and its significance

Abstract：Based on a mineralogical and petrological analysis of the Shiren Mountain block, the authors summed up the deformation-metamorphic characteristics of this block and the regularity of its changes under the influence of the Luonan-Luanchuan faulted belt. A migmatite zone is developed on the southern margin of the Shiren Mountain. In this zone, the rocks have been strongly folded and basite bands can be observed clearly. The metamorphic grade of rocks becomes weak from south to north. From the low amphibolite facies and amphibolite facies schist through gneiss to granite, dark minerals gradually decrease, showing a transitional relationship with granite. From south to north, with the increasing distance from the faulted belt, the rocks change from cataclastic to palimpsest in texture and from sheet to gneissic-palimpsest in structure. Generally, the changes assume a continuous and gradual process from strong to weak. The recrystallization type and boundary shape of quartz and the deformation of feldspar show that the deformation temperatures of the Luonan-Luanchuan faulted belt, the Shirengou Mountain block and the metamorphic rock block vary in the range of 300 ~550 ℃, belonging to the middle-high temperature condition. Under the influence of the late tectonic process, there appeared a phenomenon of non-coordination of the temperatures. The fractal dimensions of the dynamically recrystallized quartz grain boundary are in the range of 1.332~1.243 and tend to decrease with the increasing distance from the Luonan-Luanchuan faulted belt northward. Strain rates are in the range of 6.75577E-16 to 1.81072E-16, which belong to the middle strain rate condition. The strain rate and the distance from the fault belt are positively correlated with each other.