On the Origin of Ultramafic Rocks
It is argued that most alpine ultramafic bodies originated as cumulates in basic magma chambers high in the crust. From some of these chambers, magmas were intruded upward or extruded, leaving sill-like ultramafites behind. These were subsequently folded or dismembered by faulting, and, because of their high density, subsided during tectonism, to form cold, fault-enclosed intrusions. In other places, tectonism interrupted crystallization of the magmas, magma chambers were deformed, loose cumulates and hot coherent cumulate rocks slumped and slid, and hot masses, as crystal mushes and hot lenses, worked down fault zones. These hot masses were subject to high temperature recrystallization and metasomatism, as water from adjacent rocks permeated them, forming veins of pyroxenite and dunite, and intercumulus plagioclase was destroyed. Contact aureoles were formed, but some were abandoned as the cooling ultramafites descended. The Bay of Islands complex and the Great Dyke are considered to be examples of bodies whose origins as stratiform differentiates is clear but which show some of the features of alpine ultramafic masses. It is argued that Franciscan peridotites are differentiates that have been folded or have descended along faults during tectonism. It is hypothesized that ultramafites of British Columbia are complementary to late Paleozoic or Triassic volcanic rocks. Zoned ultramafic complexes of Alaska, British Columbia, and the Urals, currently believed by many to have formed by crystallization of ultramafic magmas, are re-examined. It is concluded that the central peridotites of these consist of cumulates from basic magmas and have been dropped as crude cylinders along ring fractures or are downward intrusions of ultramafic crystal mushes. Their zoning is due to subsequent metasomatism by hydrous fluids that were guided by marginal fractures. Pipes of the Bushveld Complex may have formed entirely by replacement along and adjacent to “gas chimneys.”