A MEGACRYSTIC K-FELDSPAR INTRUSION IN NORTHWESTERN NEVADA: PETROGRAPHIC AND PB ISOTOPE ANALYSES

Although conspicuous K-feldspar crystals are common in many granitic intrusions, only a small portion of these intrusions are characterized by megacrystic K-feldspars (>4 cm). Cretaceous granitic bodies in the Santa Rosa Range, Nevada include a megacrystic K-feldspar intrusion (Granite Peak Stock - GPS) that is similar to megacrystic intrusions within the Sierra Nevada batholith. Here, we present the petrography and Pb isotope analyses of K-feldspar megacrysts from this isolated intrusion.

GPS megacrysts are commonly euhedral, ranging from 1-6cm in length (up to 10cm). Microscopy reveals abundant plagioclase, biotite, quartz, apatite, and accessory phase inclusions. Many of these inclusions, especially euhedral plagioclase and biotite, are preferentially oriented with their long axis parallel to zonation boundaries. Quartz inclusions are also found along these boundaries but typically form rounded blebs and elongate stringers. Nearly all GPS megacrysts have oscillatory zoning with thicknesses ranging from <0.5mm to > 1mm. Whereas most of these zones are interpreted to reflect continuous growth, some zones show evidence of resorption and erosion. The GPS megacrysts are also sector zoned, containing sectors primarily defined by abundant plagioclase and biotite inclusions.

Microsampling of distinct regions within megacrysts was accomplished using diamond coring bits and a Dremel Tool workstation. Following acid leaching, cores removed from two megacrysts were analyzed for Pb isotopes by TIMS. Analyses reveal no core to rim heterogeneity (avg. 206/204 = 19.075 ± 0.015; avg. 207/204 = 15.670 ± 0.020; avg. 208/204 = 38.850 ± 0.063). Additionally, Pb isotope ratios from megacrysts are within error of Pb isotopes ratios collected on non-megacrystic GPS feldspars, suggesting identical sources.

Our textural observations support a magmatic origin for these megacrysts. However, the Pb isotope data do not allow distinction between a magmatic and subsolidus origin because 1) early-formed megacrysts and interstitial K-feldspars may have crystallized from the same homogeneous magma, and 2) subsolidus modification may have formed the megacrysts at the expense of interstitial K-feldspar grains, yielding K-feldspar megacrysts with the same Pb isotope composition as the non-megacrystic GPS feldspars.