COLLABORATIVE RESEARCH: Petrogenesis of the Laramie Anorthosite Complex

地块斜长岩及其伴生碱性岩的成因仍然是一个重要的问题。拉勒米斜长岩杂岩是世界上为数不多的未变质的地块斜长岩杂岩之一，到目前为止，我们对拉勒米斜长岩杂岩的研究主要集中在演化岩石上：红山、Sybille和Maloin Ranch的二长花岗岩。我们在这些岩体中发现了分异和不同污染的证据，在马洛因发现了一个额外的过程，在这个过程中，演化较少的黑云辉长岩岩浆与分化的岩浆混合在一起。我们的初步锶和钕同位素数据表明，L.A.C.的任何单元都不是简单的亏损地幔驱动熔体，但所有单元--即使是斜长岩--都在某种程度上与地壳相互作用。此外，每个演化的深成岩都具有独特的锶和钕同位素特征，这表明尽管它们的矿物化学表明它们是由相似的过程演化的，但它们是独立演化的，并在不同程度上同化了地壳物质。到目前为止，我们的工作使我们能够确定几个重要的目标：(1)我们必须建立杂岩内部差异过程的岩石化学模型，特别是关注围岩污染产生的斜长岩内的同位素和化学趋势，斜长岩和二长花岗岩之间的过渡接触，以及与围岩很少相互作用而演化的辉长岩体。(2)我们必须确定层状和未层状斜长岩的范围，确定分层的原因，并评估是否有或全部这些斜长岩是作为底辟侵位的。(3)我们将确定氧逸度和二氧化硅活度的变化是否与地壳污染有关。(4)研究复式岩脉，以确定镁铁质部分是否与斜长岩单元的残余岩浆有关。(5)我们将继续确定各单位的绝对年龄。(6)最后，我们将试图确定产生该杂岩的岩浆的起源(S)。

The origin of massif anorthosites and associated alkaline rocks remains an important problem. To date our studies of the Laramie Anorthosite Complex, one of the few unmetamorphosed massif anorthosite complexes in the world, have focussed on the evolved rocks: the monozosyenites of Red Mountain, Sybille, and Maloin Ranch. We have found evidence for differentiation and variable contamination in these bodies, and in the Maloin have identified an additional process in which less evolved biotite gabbro magma has mixed with differentiated magma. Our preliminary Sr and Nd isotopic data indicate that no units of the L.A.C. are simple depleted mantle-drived melts, but that all-- even the anorthosites--have interacted with the crust to some degree. Moreover, the evolved plutons each have unique Sr and Nd isotope characteristics indicating that, although their mineral chemistries suggests that they evolved by similar processes, they evolved independently and assimilated crustal materials to different degrees. Our work to date allows us to identify several important goals: (1) We must produce petrochemical models of the differen- tiation processes within the Complex, focussing in particular on the isotopic and chemical trends within the anorthosites that are produced by country rock contamination on transitional contacts between the anorthosite and monzosyenites, and on the gabbroic bodies that have evolved with little country rock interaction. (2) We must determine the extent of layered and unlayered anor- thosite, determine the cause of the layering, and assess whether any or all of these anorthosites was emplaced as a diapir. (3) We will determine whether changes in oxygen fugacity and silica activity are correlated to crustal contamination. (4) We will study composite dikes to determine whether the mafic portions are related to residual magmas from the anorthosite units. (5) We will continue to determine the absolute age of the various units. (6) Finally, we will attempt to determine the origin(s) of the magmas that produced the Complex.