RESIDUAL-LIQUID ORIGIN FOR A MONZONITIC INTRUSION IN A MIDPROTEROZOICANORTHOSITE COMPLEX - THE SYBILLE INTRUSION, LARAMIE ANORTHOSITE COMPLEX, WYOMING

The Sybille intrusion (=100 km(2)) is one of three large monzonitic in trusions in the 1.43 Ga Laramie anorthosite complex of southeastern Wy oming. The petrographic, geochemical, isotopic, and geophysical charac teristics of Sybille monzonitic rocks are consistent with an origin by extensive crystallization of liquids residual to nearby anorthositic cumulates (ferrodiorites) and contamination by Archean wall rocks, The exposed part of the intrusion is composed mainly of coarse-grained mo nzosyenites with abundant alkali feldspar phenocrysts, The monzosyenit es preserve mineralogical evidence for high crystallization temperatur es (>1000 degrees C), mid-crustal emplacement pressures (approximate t o 3 kbar), relatively reduced crystallization conditions (2 log units below the fayalite + magnetite + quartz [FMQ] oxygen buffer), and they crystallized in the presence of a CO2-rich fluid phase (Fuhrman et al ., 1988; Frost and Touret, 1989), The eastern monzosyenites, those adj acent to contemporaneous anorthosite, are distinguished by an anhydrou s mineral assemblage (Fo(16)-Fo(8) olivine, high-Ca pyroxene) lacking modal quartz, silica contents of <60 wt%, and relatively large Eu anom alies (Eu/Eu = 2.1 to 2.7). In contrast, the western monzosyenites, i n proximity to Archean gneisses, are distinguished by the presence of modal quartz (3% to 28%), primary hornblende, Fe-enriched fayalitic ol ivine (Fo(7)-Fo(4)), silica contents of >60 wt%, and smaller Eu anomal ies (Eu/Eu = 1.2 to 1.3), Abundant xenoliths of Archean wall rocks an d anorthosite from the adjacent intrusions in all monzosyenites attest to a stoping emplacement mechanism near the roof of the chamber. We p ropose that the monzosyenites represent a relatively thin, 0.5-1.0-km- thick, roof to a magma chamber dominated by dense ferrodioritic cumula tes at depth, Extensive, open-system fractionation of a ferrodioritic parent magma, residual after crystallization of anorthosite, produced Fe-enriched monzodioritic and/or monzonitic magma in the upper part of the chamber and complementary Fe- and Ti-rich cumulates in the lower levels, We have corroborated the production of monzonitic liquids from crystallization of ferrodiorite through a series of reconnaissance eq uilibrium-crystallization experiments, The presence of dense ferrodior itic cumulates at depth is consistent with the prominent positive grav ity anomaly associated with the Sybille intrusion (Hedge et al., 1973) , In the upper parts of the chamber, the fractionated monzodioritic an d/or monzonitic magmas eventually became saturated in alkali feldspar, Owing to density contrasts, the alkali feldspar phenocrysts floated t o the roof of the chamber, thus producing the exposed porphyritic monz osyenites. In addition, the roof of the chamber was the site of signif icant melting of Archean gneiss and, locally, metapelite, The Sr and N d isotopic compositions of the monzosyenites, with Sr isotopic ratios becoming increasingly radiogenic from east (I-Sr = 0.7059 and initial epsilon(Nd) = -2.5) to west (I-Sr = 0.7092 and initial epsilon(Nd) = - 2.6), are consistent with a 5% to 15% addition of Archean orthogneiss to a ferrodioritic parent magma that had isotopic characteristics simi lar to adjacent anorthositic rocks. The stratigraphic and compositiona l similarity of the Sybille monzosyenites to mangerites in the Bjerkre im-Sokndal intrusion of the Rogaland anorthosite complex, southern Nor way, indicates that similar open-system magmatic processes are capable of having produced high-temperature, K-rich monzonitic rocks in other Proterozoic anorthosite complexes.