A microcosm of modern crust formation: Evidence from zircon ages, Hf O and Nd Sr isotopes and bulk geochemistry of the Menglian Batholith, SE Tibet
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Batholith
Felsic
Adakite
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Crustal recycling
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The composition of much of Earth's lower continental crust is enigmatic. Wavespeeds require that 10–20% of the lower third is mafic, but the available heat-flow and wavespeed constraints can be satisfied if lower continental crust elsewhere contains anywhere from 49 to 62 wt% SiO 2 . Thus, contrary to common belief, the lower crust in many regions could be relatively felsic, with SiO 2 contents similar to andesites and dacites. Most lower crust is less dense than the underlying mantle, but mafic lowermost crust could be unstable and likely delaminates beneath rifts and arcs. During sediment subduction, subduction erosion, arc subduction, and continent subduction, mafic rocks become eclogites and may continue to descend into the mantle, whereas more silica-rich rocks are transformed into felsic gneisses that are less dense than peridotite but more dense than continental upper crust. These more felsic rocks may rise buoyantly, undergo decompression melting and melt extraction, and be relaminated to the base of the crust. As a result of this refining and differentiation process, such relatively felsic rocks could form much of Earth's lower crust.
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We studied the formation of low-initial-Sr felsic plutons by using data from the Early Cretaceous western Peninsular Ranges batholith near Escondido, California. The systematically sampled Escondido plutons have a uniformly low initial 87Sr/86Sr isotope ratio of Sri < 0.704, but a wide range of SiO2 compositions, from 46 to 78 wt%, which fall in three distinct groups: 20% gabbros, 35% tonalites, and 45% granodiorites. These low-Sri plutons are unique in having undergone one cycle of mantle melting to give basalt composition rocks, and a second cycle of arc basalt melting to give a range...
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DOI = 10.3126/hjs.v5i7.1315 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.130-131
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Adakites have a distinct chemistry that links them to melting of a mafic source at high pressure. They have been attributed to melting of subducted oceanic crust or melting of the mafic crustal roots of thick continental arcs, and are an important contrast to mantle wedge melting as a means of generating continental crust. We report the first direct evidence for the generation of adakitic melts in mafic lower continental crust, in an exhumed Cretaceous arc in the South Island of New Zealand. The lower crustal Pembroke Granulite has the bulk chemistry and partial melting textures involving peritectic garnet appropriate for a source region for an adakitic melt. The melt migrated from the area through a fracture network now filled with trondhjemitic veins. Emplacement of the melt was in the upper crust of the Cretaceous section, illustrated by the presence of coeval adakites in the upper crustal Nelson-Westland region.
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