Abstract

Chemical analyses of twenty coexisting pyroxenes, hornblendes, biotites, and ilmenites have been made from the Guadalupe igneous complex—a differentiated gabbroic intrusion which crystallized under a relatively high ∫H2O. The differentiates are similar in mineralogy and bulk chemical composition to the common calc-alkaline plutonic suite of orogenic regions.

Special attention has been devoted to the coexisting Ca-rich clinopyroxene and orthopyroxene which characterize the early gabbroic differentiates. These pyroxenes have extreme, and very uniform, contents of Ca, and this fact, together with comparisons of their Mg-Fe partition with pyroxenes from other types of rocks, suggest those from the Guadalupe crystallized at relatively low temperatures, such that the liquidus was depressed far down on the pyroxene solvus. Crystallization of pigeonite (Ca-poor clinopyroxene) was precluded under these conditions. This trend of pyroxene crystallization, the first of its kind to be established, is considerably different from the Skaergaard trend and is closely similar to variations found in pyroxenes from granulite-facies metamorphic rocks.

Hornblende appears as a primary phase early in the differentiation sequence and biotite appears in the more Fe-rich gabbroic rocks. With decreasing temperature, and possibly increasing ∫H2O, hornblende and biotite take the place of pyroxenes in the intermediate differentiates, diorite and quartz monzonite. Biotite is the sole ferromagnesian silicate in the late silicic differentiates, granite and granophyre. Hornblende and biotite from successive differentiates show enrichment in Fe relative to Mg, but variations in concentrations of elements are not nearly as regular as the variations in pyroxenes, possibly because of the range in rock types represented.

Coexisting orthopyroxene, clinopyroxene, hornblende, and biotite, in that order, have decreasing concentrations of Fe2+ and Mn but increasing A1 and Ti.

The dominant oxide phase in gabbroic rocks is ilmenite, whereas intergrowths of magne-tite-ilmenite occur in more silicic differentiates, Pyrite and pyrrhotite appear sparingly in the earliest gabbroic rocks. The ilmenites contain only about seven atomic percent hematite in solid solution, indicating ∫o2 was about 10-10 to 10-13 bars at crystallization temperatures (inferred to be approximately 1100° to 900°C) and remained at values low enough during cooling to prevent extensive oxidation. The ∫o2 is only slightly lower than that inferred for the crystallizing Skaergaard magma. Utilizing simplifying assumptions, it is shown that the Guadalupe magma, though having a relatively high ∫H2O, could have maintained a low ∫O2 if other “volatiles,” such as S2, SO2, CO2, etc., were present.

High ∫H2O and early crystallization of hornblende play an important role in the develop-ment of the calc-alkaline trend from basaltic magmas. Relatively high ∫02, is not a necessary accompanying factor. In fact, high ∫02, alone cannot produce the characteristic hydrous ferromagnesian silicates of this trend. It has yet to be demonstrated whether relatively high ∫02 is a pertinent factor in the evolution of natural magmatic sequences.

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