Feldspar Microboudinage Paleopiezometer And Its Applications To Estimating Differential Stress Magnitudes In The Continental Middle Crust (Examples From West Yunnan, China)

Feldspar, which is the most abundant crustal mineral, exhibits brittle behavior while quartz and mica deform plastically by dislocation creep, under the greenschist-facies conditions (300-450 degrees C). Pull-apart micro-boudinage, which formed by repeated cycles of tensile fractures, is a common microstructure observed in feldspar porphyroclasts embedded in the ductile matrix of quartz in granitic mylonites from the Red River, Chongshan and Gaoligong shear zones (Yunnan, China). These crustal-scale shear zones, which lie in the southeastern margin of the Tibet Plateau, have played a critical role in the southeastward lateral extrusion of the Indochina composite block in response to the Indo-Asian continental collision. Here we developed a feldspar microboudinage paleopiezometer. In each feldspar porphyroclast, the differential stresses estimated from microboudins progressively increases with sequential fracturing, depicting an evolution during exhumation of the mylonite upwards the brittle-ductile transition. The results from the feldspar microboudinage piezometer and the quartz recrystallized grain-size piezometer are largely consistent within the range of uncertainties. The microboudinage piezometer, combined with results of finite strain measurements, deformation temperature estimates, and thermochronological data, can yield the valuable information about rheological properties of naturally deformed rocks. It can also be directly used or adapted to a wide range of geological problems, for example, fractured pebbles in an unlithified matrix, chromite pods in serpentinized mantle rocks, and strong boudins in a soft material at any scale.