GEOLOGICAL NOTES EVOLUTION OF A HETEROGENEOUS, CONTINENTALLY DERIVED GRANITE: DONA INÊS PLUTON, NORTHEASTERN BRAZIU JUDE MCMURRY,2 LEON E. LONG, AND ALCIDES N. SIAL Department ofGeological Sciences, Vniversity ofTexas, Austin, TX 78713, V.S.A. Department of Geological Sciences, Vniversity of Texas, Austin, TX 78713, V.S.A. Department of Geology, Federal Vniversity of Pernambuco, Recife 50.000, PE, Brazil ABSTRACT The Dona Inês pluton, northern Paraíba, is a fine-grained, foliated, garnetiferous leucocratic granite, a product of a low degree of partial melting. SiOz values are high and restricted (70.6 to 75.5 wt %). Data trends of major oxides vs. SiOz are generally systematic, but trace element distributions are irregular. Two distinct groups of rocks (A and B) are apparent in the field, Group B crosscutting Group A along numerous internal contacts. Group B rocks have higher SiOz and lower KzO, CaO, FeO, Sr, Ba, and LREE than Group A rocks. Erratic Rb-Sr isotopic behavior provides "scatterchrons" rather than true isochrons. For Group A samples, t = 475 ± 102 Ma and initial 87SrfS6Sr= 0.7159 ± 0.0046; for Group B rocks, t = 544 ± 16Ma and initial87SrfS6Sr = 0.7119 ± 0.0009. Whole-rock 1)180is low and unsystematic, ranging from +6.5 to + 8.00/ooSMOW. For Group A rocks, REE data plot subparallel with slightly diminishing abundances with increasing SiOz. Group B samples are strongly depleted in LREE and enriched in HREE relative to Group A rocks. The Dona Inês pluton likely originated as a partial melt of metasedimentary source rocks with a volcaniclastic component. The magma, containing some refractory "restite," failed to mix completely, thus inheriting isotopic and trace element inhomogeneities from the source. After Group A magma was intruded, a fractionation marked by removal of feldspar and biotite, and by crystallization of an accessory mineral (probably allanite) substantially diminished the abundances of LREE in the remaining B liquid. INTRODUCTION A suite of igneous rocks derived by partial melting of metasediments may exhibit clear petrographic and chemical affinities while re- taining some of the heterogeneity imparted from the source. Pristine magma may be modified by crystal-melt fractionation and by entrainment of refractory source material (restite) (White and Chappell 1977). These processes produce the isotopic and trace- element variations that have been noted along transects of major batholiths which are believed to have sampled the underlying con- tinental crustal source on a large scale (Bate- man and Dodge 1970; Kistler and Peterman 1973; McNutt et aI. 1975). Although composi- tional differences tend to be more apparent I Manuscript received April 21, 1986; accepted August 13, 1986. 2 Current address: Department of Geosciences, Texas Tech Vniversity, Lubbock, TX 79409, V.S.A. [JOURNALOFGEOLOGY, 1987, vol. 95, p. 107-117] © 1987 by The Vniversity of Chicago. Ali rights reserved. 0022-1376/87/9501-0010$1.00 107 between plutons than within them, the in- homogeneity of a single pluton can be pro- nounced (e.g., Fourcade and Allegre 1981; Stormer et aI. 1980). A pluton may be inter- nally as variable as the entire batholith of which it is a part (Noyes et aI. 1983). We have documented such patterns in a small (35 km2) pluton in northeastern Brazil which, though petrographically simple, is chemically and isotopically complex. We postulate that the pluton formed from at least two distinctly fractionated pulses of a magma derived from a single heterogeneous source region. GEOLOGIC SETTING The Dona Inês intrusion, named for a small village, is a quartz-rich, in part garnetiferous pluton in northern Paraíba about 80 km from the Atlantic Coast near the Paraíba-Rio Grande do Norte border (fig. 1). The pluton appears to be part of a group of late-tectonic granites (Almeida 1971) intruded during the Brasiliano (= Pan-African) Cycle, the last major orogeny to have affected northeastern Brazil, about 700 to 500 m.y. ago (Wer- nick 1981; Brito Neves et aI. 1974). Local