Geology of the Ramona Pegmatites, San Diego County, California

Citation

Simpson, Dale Rodekohr (1960) Geology of the Ramona Pegmatites, San Diego County, California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/65DM-QT70. https://resolver.caltech.edu/CaltechETD:etd-09202006-111427

Abstract

The Ramona pegmatite district of central San Diego County, California, has been known as a source of gem garnet, tourmaline, and topaz since the early 1900's. All the occurrences of known commercial importance lie within an area of about 1.5 square miles. The dominant rocks of the district are tonalites and granodiorites of the Cretaceous southern California batholith. The nearest outcrop of older rocks, the Triassic Julian schist, is about one mile from the main group of pegmatites. Eocene Poway conglomerate caps hills and ridges in a west-trending belt that lies south of the district. The pegmatite bodies, ranging from small stringers to large dikes about 8 feet thick, are remarkably persistent along their strike. They form a pattern of subparallel anastomosing units with a general northwest strike and a westward dip of 35 to 45 degrees. The dikes occur mainly in the tonalites, and appear to have been emplaced along a well-developed set of fractures. These fractures are not systematically related to other structural features of the area, and they transect a major contact between plutons of different composition. Most of the pegmatite dikes are asymmetrically zoned, each with a footwall unit of layered aplite and a hanging-wall unit rich in graphic granite. Typical inner zones of the dikes consist of quartz-perthite pegmatite, pocket-bearing pegmatite, and albite-quartz rock. Aplite generally constitutes about one-third of the containing pegmatite body. It is a soda-rich rock, with about 40 percent of modal albite. Garnet-tourmaline layers in the aplite are roughly parallel to the footwall of the pegmatite body. Typical graphic granite, which also forms about one-third of the containing pegmatite body, has a quartz-perthite ratio of 1:3. The microcline-albite ratio within the perthite is about 2:1. Studies of mineral orientation show that the quartz rods in a single specimen of graphic granite have a common crystallographic orientation, but that this orientation is different for different specimens and is not consonant with any crystallographic law. Moreover, the c axis of quartz is generally not the axis of elongation of the rod, nor does the c axis have a systematic angular relation to the walls of the pegmatite dike. Successive surfaces cut in a block of graphic granite show many interconnections of the quartz rods. The quartz-perthite pegmatite commonly contains large euhedral crystals of perthite and ragged-edged blocks of graphic granite in an allotriomorphic groundmass consisting mainly of quartz, perthite, and albite. Albite-quartz rocks occur as fracture fillings and irregular masses within the pegmatite dikes. The albite commonly has a curved cleavelandite form. Modally these rocks have an albite-quartz ratio of 2:1. The pocket pegmatite, which ordinarily occurs in the quartz-perthite rock, consists mainly of smoky quartz with some cleavelandite and topaz. The pockets themselves form spherical voids to thin openings with irregular outlines. A period of corrosion and secondary mineralization followed crystallization of the pegmatites. The sequence of removal of minerals was quartz--->microcline--->albite, and the sequence of secondary mineralization was albite--->orthoclase (with an adularia habit)--->quartz with a typical low-temperature crystal form. Spessartite and secondary tourmaline most likely crystallized during the end stages of the corrosion or the beginning of the albite mineralization. Pseudomorphs of manganese oxide after garnet (?), as well as pseudomorphs of muscovite after tourmaline, are present in otherwise unaltered quartz-feldspar rocks. The bulk mineralogical composition of the pegmatite bodies is 30 to 35 percent for each of the minerals quartz, microcline, and albite. Muscovite, garnet, and tourmaline form about 5 percent of the total rock. The experimentally determined liquidus for a composite sample of the pegmatite under various water-vapor pressures is about 50°C, higher than the corresponding liquidus for the ternary minimum of the system KAlSi3O8-NaAlSi3O8-SiO2-H2O. Similarly, the pegmatite solidus is about 50 degrees lower than that for the ternary minimum, and it can be further lowered by the addition of fluorine-bearing minerals to the rock melt. Water is soluble in a melt of the composite sample of pegmatite to the extent of about 3.5 and 5.0 weight percent at 1 and 2 kilobars, respectively, of water-vapor pressure. Laboratory crystallization of the pegmatite melts was found to be a fairly slow process, requiring several weeks to form 1 or 2 percent of crystals. The crystals are larger and form more rapidly in melts under high water-vapor pressure and with little undercooling. Spherulitic crystal aggregates form with undercooling of 75 degrees or more. The natural pegmatites probably crystallized from a magma and a coexisting vapor that were residual from the magma forming the large composite southern California batholith. An hypothesis of crystallization in a restricted system best explains the features of the pegmatite bodies. Spatial relationships indicate that the dikes crystallized from the walls inward. Hypotheses are presented to explain the layered aplites either as products of rhythmic crystallization and crystal settling or as products of in-situ rhythmic crystallization progressing upward from the footwalls of the dikes. The rhythmic crystallization could have been a result of pressure fluctuations. Graphic granite cocrystallized with the layered aplite. All available evidence indicates simultaneous crystallization of quartz and feldspar, with the graphic texture a result of accelerated growth of one mineral alternating with accelerated growth of the other mineral. Temperature and water vapor-pressure during crystallization of the graphic granite and layered aplite were most likely about 690° to 715°C. and 3000 to 4500 bars, respectively. The graphic granite and the quartz-perthite pegmatite probably crystallized from a melt and a coexisting vapor, and the layered aplite from the melt. The other pegmatite units are thought to be products of crystallization from a hydrothermal fluid that was mainly in the form of a vapor.

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