PETROLOGY OF TERRA NOVA PLUTON, BRAZIL, AND ASSOCIATED ULTRAPOTASSIC DYKES
2018; Sociedade Brasileira de Geologia; Volume: 17; Issue: 4 Linguagem: Inglês
ISSN
2177-4382
AutoresAdejardo Francisco da Silva Filho, Philip T. Leat, R. N. Thompson,
Tópico(s)Radioactive element chemistry and processing
ResumoThe Upper Precambrian Terra Nova Pluton, situated 550 km inland from Recife, Brazil, is 220 km 2 in area and intrudes deformed metasedimentary rocks of the Pianco-Alto Brigida Mobile Belt. The pluton shows complex petrological relationships. It consists of subalkaline quartz-monzonites and quarts-syenites, and the major minerais are K-feldspars, albite, hornblende, and quartz. The pluton is intermediate in composition (Si0 2 =58.9-65.6 wt %; MgO=0.9-3.7 wt % ) and is dominantly potassic (K 2 0=3.3-5.6 wt %; K 2 O/Na 2 O=0.9-1.8). Ba (up to 2,300 ppm) and Sr (up to 1,100 ppm) are abundant in the rocks, and LREE are enriched relative to HREE (La N /Lu N = 25.6-43 .2). There is no significant Eu anornaly. Rounded autoliths with in the pluton are similar, but more maflc in composition (Si0 2 = 54.6-57.5 wt %; MgO = 4.9-6.4 wt %). A suite of dykes cut pluton and the surrounding country rocks. These dykes are varied in composition, encompassing most of the chemical range shown by the pluton and associated auto liths. The dykes are holocrystalline, peralkaline, and strongly enriched in both K 2 0 (K 2 0 = 5.3-11.4 wt %) and Ba (Ba = 2,400 ppm-l0,500 ppm), which are considered to be magmatic abundances. The dykes have similar REE and o ther trace elements and ratios to the auto liths and plutonic rocks, and the dykes and the pluton are thought to be chemically related. The Terra Nova Pluton records the fractionation of mantle-derived ultrapotassic magma from mafic to intermediate compositions.
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