Introduction

Plutons known to be of Jurassic age or of Cretaceous age, based upon U-Pb geochronology, are common in the EMNSA (pl.1; fig. 4). Other plutons are definitely or almost certainly Mesozoic, but it is uncertain whether they are Jurassic or Cretaceous. The Jurassic and Cretaceous plutons and ranges in which they crop out within the EMNSA are too numerous to list or describe individually. Rather, the descriptions below focus upon typical or relatively well-studied plutons, and upon features of special interest.

Both the Jurassic and the Cretaceous plutons within the EMNSA are small parts of magmatic belts that extended through much of the southern Cordillera and are oblique to one another (Miller and Barton, 1990; Fox and Miller, 1990; Tosdal and others, 1990). The northeast margin of the composite magmatic belt lies wthin the EMNSA in the central parts of the Ivanpah and New York Mountains.

Known Jurassic plutons and Cretaceous plutons in the EMNSA region generally differ in petrology and geochemistry. Miller and others (1982), Miller and others (1985), Howard and others (1987), and Fox and Miller (1990) have summarized the characteristics of Jurassic and Cretaceous plutonic rocks in the Granite Mountains, southern Providence Mountains, and Colton Hills of the southern EMNSA. The Cretaceous granitoids are characterized by relatively low color index, white to buff or flesh-colored feldspars, and absence of clots of mafic minerals. In contrast, the Jurassic granitoids commonly are more heterogeneous, contain less quartz, more commonly are conspicuously sphene-bearing, are more potassic, have higher color index, contain lavender, grey, or pinkish alkali feldspar, and contain clots of mafic minerals. In some places, Jurassic plutons are associated with magnetite skarn deposits or zones of extensive albitization, neither of which is documented for Cretaceous plutons. For some Mesozoic plutons for which U-Pb ages have not been determined, a reasonable inference as to a Jurassic or a Cretaceous age can thus be made from the overall petrology or composition of the plutons. Such estimates are probably most applicable to granodiorite and granite compositions, and have a lower probability of being correct for dioritic or gabbroic rocks. In the report, we follow the IUGS classification scheme (Streckeisen and others, 1973) adopted for plutonic igneous rocks. Geochronological data, much of it unpublished, for plutons in the EMNSA hint at multiple intrusive episodes for each of the Jurassic and Cretaceous groups of plutons. Many Jurassic plutons appear to be 160 to 165 Ma, but some plutons and dikes are as young as 150 to 145 Ma. Cretaceous plutons appear to belong to a late Early to early Late Cretaceous intrusive event, 90-100 Ma, and a late Late Cretaceous event, 70-75 Ma. General distingushing characteristics of Jurassic and Cretaceous plutons are maintained despite episodicity within the two groups.

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