COLLABORATIVE RESEARCH: Petrogenesis of the Laramie Anorthosite Complex

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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.