COMPOSITION AND PETROGENESIS OF OXIDE-RICH, APATITE-RICH GABBRONORITES ASSOCIATED WITH PROTEROZOIC ANORTHOSITE MASSIFS - EXAMPLES FROM THE ADIRONDACK MOUNTAINS, NEW-YORK

Mafic dikes and sheets rich in Fe, Ti-oxides and apatite are commonly associated with Proterozoic massif anorthosites and are referred to as oxide-apatite gabbronorites (OAGN). Within the Adirondacks, field evi dence indicates that during middle to late stages of anorthositic evol ution, these bodies were emplaced as magmas with unspecified liquid-cr ystal ratios. Sixty whole rock analyses of Adirondack OAGN and related rocks define continuous oxide trends on Harker variation diagrams (Si O2 = 37 54%). Similar trends exist for Sr, Y, Nb, Zr, and REE and toge ther suggest a common origin via fractional crystallization. A represe ntative parental magma (plagioclase-rich crystal mush) has been chosen from this suite, and successive daughter magmas have been produced by removal of minerals with compositions corresponding to those determin ed in actual rocks. Least squares, mass balance calculations of major element trends indicate that removal of intermediate plagioclase (appr oximately An40-50) plus lesser amounts of pyroxene account for the com positional variation of this suite and produce very low sums of the sq uares of the residuals (R(S)2 < 0.25). The extracted mineral phases co rrespond volumetrically and compositionally to those of the anorthosit ic suite, and the model succeeds in accounting for the observed OAGN t rends. The major element model is utilized to calculate trace element concentrations for successive magmas, and these agree closely with obs ervation. We conclude that, beginning with a plagioclase-rich crystal mush, the extraction of intermediate plagioclase (approximately An40-5 0) drives residual magmas to increasingly Fe-, Ti-, and P-rich and SiO 2-poor conditions characteristic of Fenner-type fractionation. The cry stallization sequence is plagioclase --> plagioclase + orthopyroxene - -> plagioclase + orthopyroxene (pigeonite) + augite. Fe, Ti-oxides beg in to crystallize near the end of the sequence and are followed by apa tite and fayalitic olivine which appears in place of pigeonite. Augiti c pyroxene becomes the dominant ferromagnesian phase in late stages of fractionation. Resultant OAGN magmas are injected into congealed anor thosite by filter pressing of liquid-rich interstitial fractions. Vary ing compositions of the dikes reflect filter pressing at different sta ges during fractionation and thereby provide information on the fracti onation history of Proterozoic massif anorthosites.