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Abstract

There is only one long traverse today, with a long approach. We will examine field relations of an accessible portion of the 90–94 Ma Lamarck Granodiorite (Coleman et al., 1995; Wenner, Gracely, and Coleman, work in progress). Examples of similar (and, in some cases, more spectacular) features are found in other plutons exposed in the high Sierra Nevada, but this is the only example we are aware of that can be examined on a day hike.

The Lamarck Granodiorite is a long, narrow map unit that extends more than 60 km along strike, but it typically is less than 5 km wide and only locally reaches its maximum width of 10 km (Fig. 21). Thus, in sharp contrast to the TIS visited on Days 1 and 2, the Lamarck is dike-like in map view. The Lamarck pluton is composed of a diverse group of intrusive rocks that range in composition from diorite to granite and have been interpreted to be related by magma mixing (Bateman and Moore, 1965; Frost and Mahood, 1987; Coleman et al., 1992).

Hathaway (2002) mapped the south-central part of the Lamarck pluton in significant detail and subdivided it into ten map units (Fig. 22). He interpreted the units to be dike-fed and, at least in part, to have been intruded along faults including the Long Lake shear zone which we will see today. The main focus of today's traverse is the Treasure Lakes mass of Hathaway (2002), a very early—perhaps the earliest—phase of the Lamarck.

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