Excess Silica Substitution in Plagioclase Grains in the Pasamonte Eucrite

Pasamonte is a clast-rich polymict basaltic breccia with O- and Cr-isotopic compositions that are resolved from those of most eucrites. It is dominated by two mafic clast types: (i) very-fine- to fine-grained, variolitic, subophitic and ophitic basalts, usually containing zoned pyroxenes; and (ii) fine- to medium grained hypidiomorphic-granular and allotriomorphic-granular microgabbros containing pyroxenes composed of augite lamellae in homogeneous pigeonite hosts. Minor clast types are fine-grained impact-melt, mafic-breccia and mafic-granular clasts; coarse matrix mineral fragments include pyroxene, plagioclase, silica, ferroan olivine and ilmenite. Our petrologic studies include determination of plagioclase compositions for the two major clast types and matrix grains, which we report here.