BH250-248
Title
BH250-248
Subject
Hornblende Gabbro
Description
Major Minerals: hornblende, plagioclase
Hornblende gabbro is a coarse-grained (phaneritic), mafic intrusive igneous rock composed primarily of hornblende (amphibole) and plagioclase feldspar. It forms from the slow cooling of hydrous mafic magma at depth, allowing well-developed, interlocking crystals to grow. The rock is typically dark in color, with black to dark green hornblende contrasting against lighter gray to white plagioclase. The presence of hornblende, rather than pyroxene, indicates crystallization under water-rich conditions, commonly associated with subduction-related magmatic arcs. In thin section, hornblende shows strong pleochroism and characteristic cleavage, while plagioclase exhibits polysynthetic (albite) twinning. This rock type is commonly found in arc-related plutonic complexes such as those in San Diego County, California, within the Peninsular Ranges Batholith, and provides important insights into magma composition, water content, and crystallization conditions in convergent margin settings.
Hornblende gabbro is a coarse-grained (phaneritic), mafic intrusive igneous rock composed primarily of hornblende (amphibole) and plagioclase feldspar. It forms from the slow cooling of hydrous mafic magma at depth, allowing well-developed, interlocking crystals to grow. The rock is typically dark in color, with black to dark green hornblende contrasting against lighter gray to white plagioclase. The presence of hornblende, rather than pyroxene, indicates crystallization under water-rich conditions, commonly associated with subduction-related magmatic arcs. In thin section, hornblende shows strong pleochroism and characteristic cleavage, while plagioclase exhibits polysynthetic (albite) twinning. This rock type is commonly found in arc-related plutonic complexes such as those in San Diego County, California, within the Peninsular Ranges Batholith, and provides important insights into magma composition, water content, and crystallization conditions in convergent margin settings.
Coverage
Location: San Diego County, California, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 248. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-248,” BH250 Mineralogy Teaching Collection, accessed April 26, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/331.