GEOCHEMICAL AND GEOCHRONOLOGICAL ANALYSIS OF THE CUCHILLO MOUNTAIN LACCOLITH, SIERRA COUNTY, NEW MEXICO

G. Michelfelder, Nncy J. McMillan
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Abstract

Recent studies of the Sierra Cuchillo laccolith and surrounding volcanic sequences have refined the geochronology and petrologic understanding of the area. New U-Pb zircon data on the laccolith, previously dated at 49 Ma by Jahns (1978), place intrusion and cooling at 38.6 ± 0.7 -1.2 Ma. The laccolith is a fine-grained granitic intrusion containing at least three pegmatite-like concentric zones of alteration formed by late-stage volatile fluids. The source of the laccolith, at least in part, is assimilated Precambrian crust. This is supported by zircon cores dated at 1.455 Ga + 15 Ma. Six volcanic packages surround Sierra Cuchillo; two have been hypothesized as being related to the laccolith (McMillan, 1978). The original interpretation of these sequences places the undifferentiated Latite-Andesite sequences directly above the Mesozoic equivalent to the Dakota Sandstone. This is overlain by the Dacite-Rhyolite sequence (Jahns, in press). U-Pb zircon dating places the Latite-Andesite sequence at 36.2 + 0.7 Ma, and the Dacite-Rhyolite sequence at 36.6 + 0.7 Ma. Geochemically the Latite-Andesite sequence is very similar to the Sierra Cuchillo laccolith containing a Sr concentration of 837 ppm, and a Nb concentration of 9 ppm. The Dacite-Rhyolite sequence contains slightly higher concentrations of Rb, Th, Nb, Zr, and Y in the basal member, but the top member is similar to Sierra Cuchillo. Many smaller intrusions surround the Sierra Cuchillo laccolith including the Willow Springs dome and the Vindicator sill. The samples from the Vindicator sill, dated at 37.7 + 0.7 Ma, have trace element concentrations very similar to Sierra Cuchillo. These samples indicate mid-Tertiary
新墨西哥州塞拉县库奇罗山laccolith的地球化学和年代学分析
最近对库奇罗山脉火山岩及其周围火山序列的研究,已经完善了对该地区的地质年代学和岩石学的认识。Jahns(1978)在lachlith上的新的U-Pb锆石数据(先前测定为49 Ma)表明侵入和冷却在38.6±0.7 -1.2 Ma。泥纹岩是一种细粒花岗质侵入体,包含至少三个伟晶岩状同心蚀变带,由晚期挥发性流体形成。溶石的来源,至少部分,是同化的前寒武纪地壳。这是由年龄为1.455 Ga + 15 Ma的锆石核支持的。六个火山包围绕着库奇罗山脉;其中两种被假设与laccolith有关(McMillan, 1978)。这些层序的原始解释将未分化的拉丁安山岩层序直接置于相当于达科他砂岩的中生代之上。它被英安岩-流纹岩层序覆盖(Jahns, in press)。U-Pb锆石定年结果表明,该地区的latite - an山岩层序为36.2 + 0.7 Ma,英安岩-流纹岩层序为36.6 + 0.7 Ma。从地球化学上看,拉丁安山岩序列与Sierra Cuchillo laccolith非常相似,Sr浓度为837 ppm, Nb浓度为9 ppm。英安岩-流纹岩层序中Rb、Th、Nb、Zr和Y的含量在基底段略高,顶部段与Sierra Cuchillo相似。库奇罗山脉周围有许多较小的侵入物,包括柳树泉圆顶和维护者喷泉。维护者号岩样的年代为37.7 + 0.7 Ma,其微量元素浓度与塞拉库奇罗非常相似。这些样品属于中第三纪
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