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Metamorphic petrology of a high‐pressure, low‐temperature subduction complex in west‐central Baja California, Mexico
Author(s) -
SEDLOCK R.L.
Publication year - 1988
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1988.tb00416.x
Subject(s) - lawsonite , geology , blueschist , metamorphism , amphibole , geochemistry , metamorphic rock , terrane , subduction , glaucophane , metamorphic facies , phengite , mafic , petrology , eclogite , facies , geomorphology , seismology , tectonics , paleontology , structural basin , quartz
The Western Baja terrane (WBt) of west‐central Baja California is an uplifted subduction complex that is divided into smaller ‘subterranes’on the basis of bounding faults and petrological differences. Each subterrane contains coherent Early Jurassic to Early Cretaceous sedimentary and mafic volcanic rocks (not melange) that have been metamorphosed under blueschist facies conditions. Key phases in metabasites and metaturbidites include jadeitic to acmitic clinopyroxene, sodic amphibole, lawsonite, aragonite, chlorite, titanite and white mica. Pressure indicators include the jadeite content of clinopyroxene and the presence of aragonite. Temperature indicators include the presence of lawsonite, the absence of greenschist facies minerals and results from vitrinite reflectance studies. Conditions at the peak of metamorphism were >8 kbar, 225–325°C for subterrane 1, 7–8 kbar, 170–220°C for subterrane 2, and 5–6 kbar, 175–200°C for subterrane 3; these correspond to cold geothermal gradients (6–9/km). Vein assemblages that include aegerine–jadeite and aegerine, albite, aragonite, lawsonite and sodic amphibole indicate uplift during continued cold conditions, probably during steady‐state subduction.