RECENTLY RECOGNIZED ORBICULAR DIORITE IN THE LANE MOUNTAIN AREA NORTH OF BARSTOW, CENTRAL MOJAVE DESERT, CALIFORNIA

An unusual occurrence of orbicular diorite was recently recognized north of Barstow while mapping in the Williams Well 7.5 ' quadrangle. Bedrock is (245-252 Ma) Permo-Triassic diorite intruded by Jurassic(?) gabbro, and cut by numerous Cretaceous granitic dikes (73-80 Ma). The orbicular rocks are localized in an area about 100 meters long and 50 meters wide, adjacent to a contact between the diorite and gabbro.

Orbicular granitoid rocks are rare but occur worldwide, and have long intrigued geologists, due to the spectacular concentric shells of rhythmically precipitated felsic and mafic layers, and radial and tangential growth of elongate crystals. They display spheroidal concentric shells of contrasting texture and mineralogy in which outward growth bands formed around a central core or inclusion within a cooling magma chamber. They occur in both volcanic and plutonic rocks.

The Lane Mountain orbicles range from 2cm to 30cm in diameter, and are solid but range from partly to completely weathered out, leaving in some cases, a hollow central cavity. The orbicles are multi-shell with granular and radial crystal growth textures, around a core similar to the country rock in which they occur. Epidote in orbicle cores suggest magma saturated in H₂O allowed epidote to form as an early magmatic phase. Multi-shell granular growth rings and radial crystal growth rings of variable composition are typical. Abrupt compositional variation of growth rings from mafic to felsic indicates crystallization from differing magma phases or episodic fluid pulses during growth of the orbs. Spiral growth rings suggest some orbs may have rotated during crystallization of growth rings.

Historically several theories have been presented for the formation of orbicles but no one theory fits every case. Recent hypothesis emphasize a magmatic origin. The best fit theory for the Lane Mountain occurrence suggests orbicle cores crystallized from water saturated magma globules that sunk within a compositionally variable low viscosity fluid crystallizing around the rotating core, which was separated from the crystallizing diorite magma in which the orbicles are enclosed.