32 Petrologic Insights on Modern and Ancient Plate Margins: The Volcanic, Plutonic, and Metamorphic Records (Posters)

Monday, 19 March 2018
8:00 AM - 12:00 PM
Lake Champlain Exhibition Hall

USING THE MELTS ALGORITHM TO MODEL MAGMA CRYSTALLIZATION WITHIN AN UPPER-CRUST PLUMBING SYSTEM IN THE WESTERN NEWARK BASIN, CENTRAL ATLANTIC MAGMATIC PROVINCE, USA

Paper number: 32-6
Program: Theme Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

We are investigating mafic igneous rocks of the Central Atlantic Magmatic Province that formed at about 201.5 Ma. In the western Newark Basin, post-magmatic tilting, folding and erosion have exposed cross-sections through the upper crust from the basalt lava flow down to 6 km. Samples of basalt and chilled margins of underlying diabase plutons approximate the parental liquid. These are used as starting compositions in the rhyolite-MELTS (v. 1.0.x) algorithm of Ghiorso and Sack (1995) and Gualda et al., (2012) to model the crystallization of the magma under upper-crustal conditions. Results will be compared with bulk rock and mineral compositions from samples throughout the plumbing system that correspond to paleodepths from about 0.3 to >5 km (~ 0.1 – 1.5 kb). All MELTS models were started at oxygen fugacity = QFM and then run without oxygen buffering, with fractional crystallization and a 10°C Temperature step. Average basalt and diabase chill compositions are very similar and MELTS runs at P = 1 kb and 2 wt.% H2O produced very similar mineral sequences and mineral and liquid compositions; however, opx crystallized in the basalt but not the diabase chill. We attempted 16 initial MELTS runs to bracket conditions at P = 0.1, 0.5, 1, and 2 kb; and 0.5, 1, 2, and 3 wt.% H2O, but some runs were unsuccessful due to initial water contents above saturation. In successful runs, olivine composition and the behavior of augite, spinel (magnetite), and rhombohedral-oxide (ilmenite), are very similar. By contrast, P and water content cause variations in liquidus T (higher at higher P and lower H2O) and liquidus phase: opx (En83) at P = 1-2 kb and 0.5-1% H2O and oliv (Fo82-83) at lower P and higher H2O. Compositions of pigeonite and plagioclase also vary with water content and P: pig has higher Mg# and plag has lower An at higher P and lower H2O. MELTS runs predict late Fe-rich oliv (Fo31-19) which has not yet been identified in thin section. The water saturation T in the liquid is lower at higher P or lower initial H2O, and saturation was not reached before the runs ended with 0.5% initial H2O at 1 and 2 kb. Additional MELTS runs will more narrowly bracket P-T-H2O conditions and will be used to test if different mineral compositions and sequences observed in the samples are due to the effects of P and H2O or require different magma compositions.

Geological Society of America Abstracts with Programs. Vol. 50, No. 2

doi: 10.1130/abs/2018NE-311161

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