The Lakeview Mountains Pluton, Southern California Batholith Part I: Petrology and Structure

Abstract

The Lakeview Mountains pluton, a part of the southern California batholith, is a steep-walled body with a tear-shaped ground plan, exposed discontinuously over an area of 100 to 130 km 2 . It is located at an abrupt local deflection of the regional structural grain and is concordant with planar structures in the enclosing rocks. These are gabbro, granitic rocks, mixed rock composed of granitic and metamorphic rocks, and regionally metamorphosed rocks of the cordierite-amphibolite facies. Most of the adjacent granitic rocks are older than the Lakeview Mountains pluton, although some may be of the same age. The pluton is almost entirely coarse-grained hornblende-biotite quartz diorite that lacks potassium feldspar. Schlieren are ubiquitous, and small lenticular inclusions, common throughout the pluton, parallel the schlieren. The schlieren, which make the rock extremely inhomogeneous on a small scale, geometrically fall into three orientation groups. One group, the most pronounced, is concordant to the outline of the pluton and planar structures in the adjacent rocks; the other two, discordant with respect to the shape of the pluton, strike northeast and northwest. Some outcrops exhibit schlieren intersecting at a large angle, and mesoscopic fabric analysis indicates that mineral orientation of much of the massive-appearing rocks is complex on the scale of an outcrop. Granitic pegmatite dikes, masses of hypersthene gabbro, and both mafic and leucocratic quartz diorite are concentrated in the geometrically and structurally deduced center of the pluton. It is concluded that the concordant schlieren resulted from outward growth of the pluton and the discordant sets in response to regional forces. The concordance between the outline of the pluton and its wall rocks and the abrupt deflection of the regional structural grain resulted from the outward growth of the pluton. The pegmatite represents the fugitive constituents of the pluton-forming magma, and the sites of pegmatite concentration in the wider part of the tear-shaped body mark the last portions of the pluton to crystallize.