Petrology and F and CL systematics of Rattlesnake Mountain and Bone Springs Sills, Trans-Pecos Magmatic Province, Texas
Schrader, Christian Michael
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This study investigates the petrogenesis and differentiation history of Rattlesnake Mountain Sill (RMS) and Bone Springs Sills (BSS) from the Trans-Pecos Magmatic Province (TPMP). Both are nepheline-normative shallow intrusions. I conducted piston-cylinder experiments on a fine-grained sample of RMS, chosen to represent the benmoreitic composition of the intruding magma. Experimental conditions spanned 8 to 20 kb with 0, 2, 4, and 8% added H2O. Garnet is common above 15 kb with <4% added H2O. Apatite and ilmenite are nearly ubiquitous, and biotite, amphibole, and clinopyroxene are common. No run products contain olivine, indicating that the RM magma is not a primary mantle melt and that it last differentiated at pressures less than the experimental pressures. The melt (glass) compositions show that it is possible to reach silica saturation from undersaturated compositions with biotite rather than amphibole fractionation as the primary agent. Evidence is insufficient to claim that this occurred in the TPMP, but it warrants consideration for provinces that have both silica-saturated and –undersaturated rocks. Bones Springs Sill differentiated from tephritic magma to a bimodal monzogabbro-monzosyenite suite. Monzosyenite liquid was generated by ~50% crystallization. Most of the liquid crystallized in the interstices of the monzogabbro but some segregated into discrete monzosyenite bodies in the upper 1/3rd of the sill. The most felsic monzosyenites were generated by an additional ~8% crystallization. Apatite and biotite compositions record Cl loss concurrent with the separation and segregation of the differentiated monzosyenite liquid. This is probably related to the exsolution of a volatile phase during which volume expansion ruptured the crystalline framework and allowed migration of the interstitial liquid. Devolatilization caused an attendant loss of Cl by its partitioning into the vapor phase. Apatite and biotite in the monzosyenite-rich upper portion of BSS have the highest F/Cl in the sill as a result of F increase with differentiation and Cl loss to vapor exsolution. An apatite-biotite mineral pair included in Ti-magnetite from this section records a temperature of 1093 ± 103 C. These minerals have high F/Cl, suggesting that their inclusion postdates devolatilization and segregation of the residual liquid.