PLAGIOCLASE FORENSICS AS A TOOL TO DETERMINE THE NATURE OF GABBRO: MANTLE-MELT OR CRYSTAL CUMULATE?

Gabbros may represent mafic melts and their composition can be used to infer a mantle source; however, quite often gabbros in arcs are cumulates of plagioclase and mafic minerals from more intermediate composition magmas only appearing to have crystallized out of basaltic melt. Mineral accumulation may or may not be apparent texturally or from bulk compositions alone, which is why we use microstructures in combination with plagioclase chemistry to test if the analyzed gabbros are truly representing mafic melts derived directly from the mantle, or if they formed from mineral accumulation processes.

We used petrography, whole rock chemistry, and electron microprobe and LA-ICPMS mineral chemistry of six gabbro samples from three Cretaceous arc segments: the western Peninsular Ranges Batholith (wPRB) of California, the Peruvian Coastal Batholith (PCB), and the Chilean Coastal Batholith (CCB). These data are used to identify any evidence for cumulate textures and other recorded magma processes (fractional crystallization and mixing). Whole rock compositions and plagioclase geochemistry are integrated to reconstruct the liquid in equilibrium with plagioclase crystals. We also investigated how the modeled liquid composition compares with that represented by the whole rocks sampled.

Five of the studied samples have cumulate microstructures and at least two plagioclase crystal populations. Plagioclase core compositions are An_95-87 and 43-46 wt% SiO₂ (Green Acres-PRB), An_66-35 and 49-59 wt% SiO₂(San Marcos-PRB), An_80-60 and 44-50 wt% SiO₂ (Pampahuasi-PCB), An_76-47 and 49-55 wt% SiO₂ (Old Gabbro-PCB), An_81-54 and 46-54 wt% SiO₂ (La Campana-CCB), and An_54-31 and 53-60 wt% SiO₂ (Caleu-CCB).

Calculations of melt compositions in equilibrium with plagioclase indicate that the Green Acres gabbro contains plagioclase crystals that are in equilibrium with its bulk rock composition, but the other five gabbros have plagioclase compositions in equilibrium with more felsic melts. Magma mixing and crystal accumulation explain plagioclase in disequilibrium observed in the gabbro cumulates. Certainly, evaluating plagioclase compositions to study the nature of gabbro is an important part of determining what the rocks really represent before petrogenetic interpretations are made.