ROCHAS VULCÂNICAS ALCALINAS DA FORMAÇÃO QUIXABA DE FERNANDO DE NORONHA, PE: QUÍMICA MINERAL E LITOGEOQUÍMICA

Abstract

The lava flows of the Quixaba Formation (4,2 - 1,5 Ma) are olivine melanephelinites (OLM), olivine melilite melanephelinites (MEM) and pyroxene melanephelinites (PIM), with levels of pyroclastic rocks and relatively thin basanite flows (BAS). Thick BAS sequences occur in Baía do Sancho and in the São José, Cuscuz and de Fora islets. Olivine compositions (m# = 85 - 75) in melanephelinites and most BAS match those of clinopyroxenes (mg# 86 -74), indicating equilibrium crystallization. The São José and Cuscuz basantitic olivine (mg# 72 - 70) and clinopyroxene (mg# = 79 -74) compositions are similar and denote more differentiated rocks. In MEM, the amount (> 10% to <2%) and composition of melilites are variable. The melilite-rich rocks have abundant olivine, with melilites poorer in MgO and richer in Al 2 O 3 . Ba, Ti- (and F) rich phlogopite and biotite are late minerals with typical chemical features in each rock type. The Si0 2 content is an indicator of the chemical differences between melanephelinites: -MEM: 37-39 wt.%; OLM: 38-40 wt.%; PIM: 40-43 wt.%-. BAS have 45-46 wt.% Si02. The increase in silica is accompanied by increase in Al 2 O 3 and decrease in MgO, Ti0 2 , Fe 2 O 3total , CaO, MnO, Cs, Y and REE. Trace element ratios show important discrepancies among the melanephelinite types: MEM e OLM: La/Nb = 0.6-0.7; Zr/Nb = 2.7-3.2; PIM: La/Nb = 0.7-1.0; Zr/Nb = 2.6-5.7. The amount of trace elements and the incompatible element ratios are variable in BAS. The data presented in this paper denote the existence of crystal fractionation processes, for instance between PIM and the thin BAS flows in the lava sequence. On the other hand, the high amount of incompatible elements in the melanephelinites and the differences in element ratios in the several rock types, point to an origin from distinct parental magmas, derived from either different mantle sources, or different degrees of partial melting of the same source.