CONTACT METAMORPHISM OF MAGNESIAN LIMESTONES AT CRESTMORE, CALIFORNIA

Abstract

The contact-metamorphic rocks at Crestmore, California, occur between magnesian marbles and a plutonic mass of quartz diorite (Bonsall tonalite) and between the same marbles and a relatively small pipelike hypabyssal mass of quartz monzonite porphyry. The marbles are preserved as two crudely lenticular bodies about 400 and 500 feet thick, respectively, that occur as screens in the quartz dioritic intrusive rocks of the southern California batholith. Both bodies, which are very similar petrographically, are composed of alternating layers of predazzite and coarsely crystalline calcite marbles and are nearly free of silica, alumina, iron, and the alkalies. The quartz diorite that engulfed the marbles is contaminated locally very near the contacts where minor monzonitic and gabbroic variants are present. Its exomorphic effects on the carbonate rocks consist mainly of: (1) the formation of metasomatic silicate contact rocks that generally are less than 1 foot thick and are composed of diopside, wollastonite, and grossularite; and (2) the conversion of nearly all the magnesium-bearing carbonate beds to periclase marbles, which subsequently were altered to predazzites. In contrast, the younger quartz monzonite porphyry, which was injected into the upper or Sky Blue marble unit, is nearly all contaminated as a result of reactive assimilation of marble. Moreover, its exomorphic silicate aureole is as much as 50 feet thick and contains the numerous complex mineral assemblages for which Crestmore is famous. Several lines of evidence indicate that much of this aureole was formed prior to the final consolidation of the porphyry intrusive mass. The thicker parts of the silicate contact aureole that surrounds the quartz monzonite porphyry exhibit the following well-defined zonal distribution of mineral assemblages, as traced outward from the intrusive body: (1) a garnet zone that is composed of grossularite and lesser amounts of wollastonite and diopside; (2) a zone characterized by only one mineral, idocrase; and (3) a zone in which monticellite is the most abundant mineral, but in which there are various amounts of clinohumite, cuspidine, ellestadite, forsterite, melilite, merwinite, perovskite, spinel, spurrite, tilleyite, and xanthophyllite. This zonation of mineral assemblages reflects a corresponding zonation in the bulk chemical composition of the rocks, as shown by changes in the ratio of metasomatic to indigenous constituents (Si + Al + Fe/Ca + Mg) from 0.66 in the monticellite zone, through 1.15 in the idocrase zone, to 1.62 in the garnet zone. There also is a strong tendency toward a mineralogical zonation within the monticellite zone, such that clinohumite, forsterite, spurrite, and spinel are concentrated in the silica-poor and calcite-rich outer part, and merwinite, cuspidine, and melilite in the more silica-rich inner part. Hence, a sequential occurrence of mineral assemblages and a change in bulk chemical composition can be traced inward toward the intrusive mass from un-metasomatized marbles. Moreover, textural features reveal a corresponding paragenetic sequence in which the more highly metasomatized assemblages appear to have formed at the expense of those that were less metasomatized. Available evidence indicates that: (1) the contact-metamorphic mineral assemblages at Crestmore and their zonation are largely the compositionally controlled products of silica, alumina, and iron metasomatism of relatively pure magnesian limestones; (2) temperatures of 625°C. or higher were reached prior to the introduction of silica into the present monticellite-zone rocks; and (3) the so-called “high-temperature” assemblages, such as monticellite, spurrite, and melilite, formed directly from the magnesian marbles without the intervention of “lower-temperature” steps that involve diopside, wollastonite, and grossularite. Therefore, it is proposed that contact metamorphism at Crestmore should be viewed as progressive metasomatism with consequent decarbonation at elevated temperatures rather than as progressive decarbonation attendant simply upon rising temperature.