REGIONAL AND CONTACT METAMORPHISM IN THE CACTUS FLAT AREA, SAN BERNARDINO MOUNTAINS, CALIFORNIA

Regional prograde metamorphism of the Wood Canyon, Zabriskie, and Carrara Formations, exposed in and around Cactus Flat in the San Bernardino Mountains of southern California, occurred at the upper greenschist to lower amphibolite facies and was synchronous with faulting and folding. Textures include foliated and bent grains with serrated articulations. Maximum temperatures were approximately 560°C. Contact metamorphic effects due to emplacement of Mesozoic intrusions are overprinted on these regionally metamorphosed rocks.

Intrusion of late Mesozoic diorite caused upper pyroxene hornfels to lower sanidinite facies contact metamorphism. Effects of diorite emplacement include slaty cleavage, folds and disruption of sedimentary structures, and increased grain size. Critical minerals in schist beds within the Carrara Formation are andalusite, sillimanite, and cordierite, and critical minerals in the marble beds are diopside, forsterite, wollastonite, and garnet. Maximum temperatures were between 735-775°C in the contact aureole. As diorite began cooling, hydrothermal fluids caused hydrous calc-silicates to replace the anhydrous cal-silicates. Continued cooling added additional hydrothermal fluids, and spinel, phlogopite, meionite, melilite, and gehlenite crystallized. Fluorine metasomatism produced humite, clinohumite, and chondrodite.

The final stage of metamorphism occurred as granitic intrusions caused static, retrograde metamorphism in the albite-epidote-hornfels facies at temperatures of about 350-450°C. Recrystallization caused increase in grain size and forceful intrusion locally folded beds. Grains straightened, articulation smoothed, and minerals grew across foliation. Low temperature, retrograde reactions occurred in the Carrara Formation as calcite and chlorite replaced higher temperature minerals. Potassium metasomatism occurred, which caused growth of muscovite porphyroblasts and replacement of andalusite by muscovite.

Contact metamorphism created by the biotite granite is overprinted on the contact metamorphic aureole of the diorite. This relationship supports Alpert's (2006) conclusion that diorite intrusion preceded granite intrusion at Cactus Flat. Time between intrusions allowed sufficient cooling for this overprinting to be recorded.