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Diorite segregations in gabbro; geochemical characteristics and conditions for origin assessed at diorite-gabbro contacts
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Diorite segregations in gabbro; geochemical characteristics and conditions for origin assessed at diorite-gabbro contacts
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Journal Article
Diorite segregations in gabbro; geochemical characteristics and conditions for origin assessed at diorite-gabbro contacts
2009
Overview
Silicic segregation veins in the Basement Sill, Dry Valleys, Antarctica, represent fracture-filling liquids differentiated from mid-Jurassic Ferrar tholeiitic basalt magmas. Geochemical and mineralogical characterizations for several of these veins and for their host gabbros within centimeters of sharp contacts with the veins provide information about silicic liquid produced from basalt in closed systems. The Basement Sill silicic veins are coarse- to pegmatite-textured diorites (∼60 wt% SiO2; 1.6%-2.6% MgO) composed of Fe-rich clinopyroxene (cpx; Fs20-60) and orthopyroxene (and pigeonite), ∼An50-60 plagioclase, and ∼20-30 vol% mesostases of micrographic quartz + alkali feldspar (∼Or80-90). The host gabbros (52-54 wt% SiO2; 5.5%-9.2% MgO) within ∼2 cm of veins contain pyroxene and feldspar with compositions that range from overlapping those in the diorite veins to those closer to characteristic of gabbro (e.g., cpx ∼Fs20; ∼An60-80) but unlike the more primitive mineral compositions representing the Basement Sill as a whole (e.g., ∼Fs15). The gabbros also contain interstitial micrographic quartz + alkali feldspar. Evolved minerals and quartz + alkali feldspar in gabbro at vein contacts are signatures consistent with evolved interstitial liquids having migrated through the sill's solidification zones to fill fractures formed by sag/collapse of roof-side solidification zones. MELTS software crystallization (at fO2 FMQ) of the sill magma (marginal chill zone as proxy), mass balance by linear regression, and Rayleigh fractionation all show that diorite forms after ∼72% at ∼1070°C but that it only generally resembles the diorite veins. Compositions that more fully resemble the actual segregations appear to require more than fractional crystallization, such as dioritic liquids admixed with up to ∼10% of the minerals in assemblages they crystallize. That is, evolved interstitial liquids produced from ∼70%-75% crystallization (e.g., SiO2 ∼60-65 wt%) over a volume of solidification zone framework and purged into fractures to crystallize \"dioritic\" pyroxene and plagioclase can produce varying diorite compositions and modes from place to place by admixing, a liquid-crystal process reasonable to expect for liquids purged into fractures.
