Geology of 1. 7 GA ( ) Baldwin gneiss in the Baldwin Lake type area, San Bernardino Mountains, southern California

Conference · Wed Mar 31 23:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5423093

Barth, A P ^[1]; Ehlig, P L ^[2]; Wooden, J L ^[3]

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1. Indiana/Purdue Univ., Indianapolis, IN (United States). Dept. of Geology
2. California State Univ., Los Angeles, CA (United States). Dept. of Geology
3. Geological Survey, Menlo Park, CA (United States)

Precambrian gneisses in the San Bernardino Mountains were first identified and described in the vicinity of Baldwin Lake by Guillou (1953). Five lithologic units mappable at 1:24,000 scale are recognized: biotite [+-] muscovite quartzofeldspathic gneiss, amphibolite, pyroxene metagabbro, augen gneiss, and biotite [+-] muscovite granitic gneiss. Baldwin gneiss with this L < S tectonite fabric is unconformably overlain by latest Proterozoic, upright, greenschist/hornfels facies quartzite (Big Bear Group). North and northeast of Baldwin Lake, the gneissic fabric is rotated toward the northwest, subparallel to the Doble fault. Along this fault, Baldwin gneiss is structurally underlain by overturned Paleozoic quartzite and marble (Zabriskie Quartzite and Carrara Formation). Regional relations suggest that the Doble fault is a northeast-directed basement thrust fault of pre-Late Cretaceous age, and may be contemporaneous with late Paleozoic deformation and metamorphism of Paleozoic rocks further west in the range. Field relations suggest that Baldwin gneiss in its type area largely retains Proterozoic fabrics and mineral assemblages, despite marginal Phanerozoic reworking. Silver (1971) reported a U-Pb zircon age of ca. 1,730 Ma for Baldwin augen ( ) gneiss, from an unknown locality, and Miller and Morton (1980) reported Late Cretaceous mica K-Ar ages from a sample of augen gneiss. Preliminary Pb isotopic ratios in galena, feldspar and whole rock samples of Baldwin gneiss, and feldspars in Mesozoic plutons suggest isotopic affinity to the Mojave crustal province of Wooden and Miller (1990).

• OSTI ID: 5423093
• Report Number(s): CONF-9305259--
• Journal Information: Geological Society of America, Abstracts with Programs; (United States), Journal Name: Geological Society of America, Abstracts with Programs; (United States) Vol. 25:5; ISSN GAAPBC; ISSN 0016-7592
• Country of Publication: United States
• Language: English

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