Petrology of the high-Mg tonalites and dioritic enclaves of the ca. 2130 Ma Alto Maranh茫o suite: Evidence for a major juvenile crustal addition event during the Rhyacian orogenesis, Mineiro Belt, southeast Brazil

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• 作者：Lu铆s Ant么nio Rosa Seixas ; Jacques-Marie Bardintzeff ; Ross Stevenson ; Bernard Bonin
• 关键词：Paleoproterozoic ; 2130  ; Ma ; Mantle wedge melting ; High-Mg tonalites and diorites ; Brazil
• 刊名：Precambrian Research
• 出版年：November, 2013
• 年：2013
• 卷：238
• 期：Complete
• 页码：18-41
• 全文大小：7510 K

文摘

Combined field observations, petrography, mineral chemistry, geochemistry, Nd isotopes and U-Pb zircon data are presented for the ca. 2130 Ma Alto Maranh茫o suite. The suite, composed by allanite-bearing biotite hornblende tonalites and commingled dioritic mafic magmatic enclaves (MME), occupies an estimated area of >300 km² in the southern Quadril谩tero Ferr铆fero region, Minas Gerais, Brazil. Tonalites and commingled MME are medium-K rocks, with high-Mg (Mg-number 鈮?#xA0;0.46) and high-Cr (鈮?5 ppm) contents. They are also enriched in large-ion lithophile elements (LILE, Sr, Ba), some high-field-strength elements (HFSE, Th, U), and light rare earth elements (REE), but depleted in Nb, Ta, and heavy REE. T_DM model ages for tonalites and MME are similar and range from 2.3 to 2.4 Ga. The 蓻_Nd(t) values are grouped around chondritic values (=0 卤 1.0). Field relations and geochemical data indicate that tonalites and commingled dioritic MME crystallized as synchronous independent magma pulses, with limited mixing. Both magmas are compositionally equivalent to high-Mg andesites in modern subduction zones, implying that the suite was issued from the partial melting of a mantle wedge metasomatised to different degrees by slab-derived melts. Geobarometry (aluminium-in-hornblende) indicates crystallization at mid-crustal depths (i.e. 0.59-0.42 GPa). The igneous suite evolved by fractional crystallization of hornblende, biotite and accessory minerals (zircon, apatite, allanite and Fe-Ti oxide). The ubiquitous occurrence of hornblende and allanite in tonalites and dioritic MME suggests that this mineral pair was largely responsible for the observed changes of light REE/heavy REE and Eu/Eu* ratios within the members of the suite. The geographical extent (>1000 km²) and Paleoproterozoic ages of the Alto Maranh茫o suite (ca. 2130 Ma) and associated granitoids of the Mineiro Belt (2.1-2.2 Ga) constitute a major event of juvenile addition to the continental crust in the context of the southern S茫o Francisco craton, and the ca. 2.2-2.1 Ga Rhyacian orogenesis of the South American platform. The age and general geochemical characteristics of the Alto Maranh茫o suite and its emplacement after a ca. 2.35 Ga juvenile, tholeiitic-source derived high-Al TTG suite, indicate a tectonic setting akin to that of Late-Archaean high-Mg granitoids.