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The San Joaquin Geological Society has no formal dues, and our activities are open to anyone interested in geology.
May 12th DINNER MEETING ABSTRACT
Dr. Wallace Kleck
I was introduced to pegmatites in 1959 in an advanced petrology class; however, I did not have an opportunity to directly examine a pegmatite body until about ten years later in Southern Riverside Co., CA. If you so desire, there are at least four pegmatite bodies I can direct you to in the nearby Sierras—ask.
Previous to 1960, pegmatites were defined as any very coarse-grained igneous rock. With the publication of a widely-used, advanced-petrology text (Turner and Verhoogen, 1960), the basic (lower silica-content) igneous rocks were ‘removed’ from the definition of a pegmatite. As well, Turner and Verhoogen (1960) were the first to emphasize the great importance and complexity of water in pegmatitic processes.
Water and igneous differentiation have become the important substance and process in pegmatite formation. Differentiation continues during the formation of the final solid, pegmatite body. Water continues to be a difficult to quantify and understand substance. The following three ‘things’ will be used to illustrate some of the processes and complexities that we think we currently understand—Rb, three Rare-Earths (La, Ce, Nd), and B.
Layered-aplite pegmatitic intrusives were defined as a particular class of pegmatite by Jahns and Tuttle, (1963). These form as near horizontal dikes with a coarse top, an aplite bottom, and a non-centered core zone. I had an opportunity to examine one of these in some detail (Kleck and Foord, 1999). They represent one of the more difficult to explain types of pegmatite bodies; they currently are the source of several models and arguments (see for example Webber and others, 1999 or London, D., 2008).
Jahns, R. H. and Tuttle, O. F. (1963) Layered pegmatite-aplite intrusives, Mineralogical Society of America, Special Paper 1, pp. 78-92
Kleck, W. D. and Foord, E. E. (1999) The chemistry, mineralogy, and petrology of the George Ashley Block pegmatite body, American Mineralogist, v. 84, pp. 695-707
London, D., (2008) Pegmatites, Canadian Mineralogist, Special Publication 10
Turner, F. J. and Verhoogen, J. (1960) Igneous and metamorphic petrology, second edition, McGraw-Hill, New York, Toronto, London
Webber, K. L., Simmons, W. B, Falster, A. U. and Foord, E. E. (1999) Cooling rates and crystallization dynamics of shallow level pegmatite-aplite dikes, San Diego County, California, American Mineralogist, v. 84, pp. 708-717
Résumé for Dr. Wallace D. Kleck
Geologic Evolution of the Southwestern Sierra Nevada-San Joaquin Basin Transition-An Excursion to Some Critical Exposures
Jason Saleeby and Zorka Saleeby Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena CA 91125
The goal of this one day field trip is to bring to light a number of critical relationships that we have discovered in regard to the Neogene-Quaternary history of the eastern San Joaquin Basin by the tracking of relationships into the Basin margin that we have discovered during multi‐decadal structure mapping and petrologic‐geochemical studies of the southern Sierra Nevada‐Tehachapi Mountains basement uplift. In published papers and in manuscripts that are in preparation we show that the Neogene-lower Quaternary section of the Basin margin extended nonconformably eastwards across recently re-exhumed Sierran basement for a considerable distance, and that strata of the Basin margin were in continuity with strata of a significant Miocene basin (termed the Walker graben) that covered most of the southern Sierra Nevada until medial Pliocene time. Neogene basin development, reorganization of principal depocenters, and partial exhumation phases are recognized to have been forced by three distinct tectonic regimes: 1) Early and Middle Miocene opening of the Pacific-Farallon slab window; 2) Late Miocene initiation of the eastern Sierra escarpment system, and derivative westward tilting of the Sierra‐Great Valley basement surface; and 3) Late Pliocene‐Quaternary delamination of mantle lithosphere from beneath the southern Sierra and Great Valley region. Some of the specific features that we will focus on in the field include: 1) the Neogene‐Quaternary southern Sierra fault system, which consists of numerous topographically and bathymetrically significant high‐angle normal and transfer faults that cut across the entire southern Sierra, and extended as growth structures into the eastern San Joaquin Basin; 2) Early and Middle Miocene chronostratigraphic markers that extend from the Walker graben into the eastern San Joaquin Basin; 3) evidence for Early to Middle Miocene rapid normal fault growth along eastern Basin exposures; 4) multiple phases of profound sediment re-‐dispersal from the Walker graben into the southern San Joaquin Basin, and then from uplifts along part of the Basin margin into the Maricopa and Tulare sub-basins; and 5) the development of the lower Kern River gorge and the Kern gorge fault scarp.
Field Trip limited to the first 25 people.