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Ana Maria Dreher, CPRM - Geological Survey of Brazil (Brazil)
Leda Maria Fraga, CPRM - Geological Survey of Brazil (Brazil)
Heitor Grazziotin, CPRM - Geological Survey of Brazil (Brazil)
Jean-Michel Lafon, Laboratory of Isotopic Geology of the Pará Federal University (Brazil)
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Dikes, lava flows and pyroclastic deposits of calc-alkaline lamprophyres were found during a mapping program undertaken by the Geological Survey of Brazil (CPRM) in the Serra do Tepequém sheet, northern Brazil. These rocks were named Serra do Cupim Lamprophyre (SCL), including spessartites and other porphyritic rocks, such as microdiorites and andesites, of similar composition and occurrence. The mapped area is dominated by granites, volcanics and basement gneisses of Orosirian age. These units are cut by the Avanavero mafic dikes and by the SCL.
The spessartite and microdiorite dikes are thin (<1m) and widespread. Lavas and pyroclastic andesitic and spessartitic rocks crop out in small areas in the north of the sheet. The SCL rocks are magnetic and greenish gray to black in color. Dike rocks are massive, whereas effusives show vesicles, oriented phenocrysts, and brecciated structures. The spessartites contain strongly zoned phenocrysts of green-brown hornblende and, less commonly, augite. Corroded plagioclase xenocrysts are rare. The groundmass is essentially made up of plagioclase and amphibole. Quartz, K-feldspar, biotite, Ti-magnetite, apatite, epidote, chlorite, carbonate and sericite are also normally present. The microdiorites and andesites are similar to the spessartites, except for additional euhedral plagioclase phenocrysts up to 1cm in length. Broken phenocrysts and scoria or pumice fragments contained in part of the andesites and spessartites attest their pyroclastic origin.
The SCL unit consists of subalkaline, metaluminous, medium-to high-K calc-alkaline rocks that plot into the andesite and basaltic andesite fields in the TAS diagram. The SiO2 and alkalis contents vary from 52.09 to 59.85wt% and from 4.01 to 6.35wt%, respectively, with K2O/Na2O ratios of 0.41 to 0.93. The Mg# values are in the 42.30-62.04 range and Ni contents (18-70 ppm) are low. The LREE are enriched relative to the HREE (La/YbN 7.10-10.01) and negative Eu anomalies are discrete to absent. The multielemental diagram indicates the SCL rocks are enriched in incompatible elements relative to the chondrite and are depleted in Nb-Ta and Ti, displaying a strong fractionation of the HFSE relative to the LILE. This pattern suggests the participation of continental sublithospheric mantle sources, previously enriched by subduction processes, in the generation of the analyzed rocks.
Geochronological analyses performed by the Pb evaporation method on zircon from a SCL andesite sample were hindered by the small number and minute size of the zircon crystals. Nevertheless, two analyzed grains gave Statherian ages of 1766±6 Ma and 1735±7 Ma, respectively. These datings, although few in number, are consistent with the late emplacement of the lamprophyres, cutting the Orosirian units of the area. In fact, the lamprophyres are close in age to the 1782 ± 3 Ma old Avanavero mafic dikes, suggesting they are similarly related to crustal extension in an intraplate setting.
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