Nepheline syenites to syenites and granitic rocks of the Itatiaia Alkaline Massif, Southeastern Brazil: new geological insights into a migratory ring Complex

ABSTRACT: The Itatiaia Alkaline Massif comprises some of the largest meso-cenozoic alkaline igneous occurrences in Brazil, covering over 215 km2 between the states of Minas Gerais and Rio de Janeiro. It appears as an elongated, 30-km long and 4.5-11.5-km wide SE-NW-trending body emplaced along accommodation zones of the Continental Rift of Southern Brazil, intruding metapelites, orthogneiss and granites of the Brasília and Ribeira fold belts. New data and geological mapping suggest that the massif evolved from a migratory magmatic center manifested as ring structures and successive moon-shaped intrusions from SE to NW in three sectors of distinct lithological and geomorphological characteristics: Southeastern, Central and Northwestern. The lithological variants in the three sectors present themselves as discrete intrusive bodies, comprising feldspathoid-bearing and quartz-bearing syenites, porphyritic to breccioid trachytes, granite, monzonite, gabbro and trachybasalt, which can be grouped into 21 units. These rocks occur in five petrographical sets: plagioclase-free nepheline syenites, plagioclase-bearing nepheline syenite and pulaskites, nordmarkite-granite series, anti-rapakivi associations and basic rocks. Sin- to tardi-plutonic dykes of nephelinites, phonolites, trachytes and rhyolites are also present.

摘要：伊塔蒂亚亚碱性杂岩体（Itatiaia Alkaline Massif）是巴西境内规模最大的中新生代碱性火成岩体产出地之一，分布于米纳斯吉拉斯州与里约热内卢州交界区域，总面积超过215平方千米。该杂岩体呈拉长状，长30千米、宽4.5~11.5千米，整体沿南东-北西走向展布，侵位于巴西南部大陆裂谷的调节带（accommodation zone）内，侵入巴西利亚造山带与里贝拉造山带的变泥质岩、正片麻岩及花岗岩体。最新实测数据与地质填图结果表明，该杂岩体起源于一个迁移式岩浆中心，表现为环状构造与依次发育的月状侵入体，从南东向北西可划分为东南、中部与西北三个具有独特岩性与地貌特征的分区。三个分区内的岩性变体均以独立侵入体形式产出，涵盖含似长石正长岩（feldspathoid-bearing syenites）、含石英正长岩（quartz-bearing syenites）、斑状至角砾状粗面岩（porphyritic to breccioid trachytes）、花岗岩（granite）、二长岩（monzonite）、辉长岩（gabbro）与粗面玄武岩（trachybasalt），可归为21个填图单元。上述岩石可划分为5类岩相组合：无斜长石霞石正长岩（plagioclase-free nepheline syenites）、含斜长石霞石正长岩与普拉斯基特岩（plagioclase-bearing nepheline syenite and pulaskites）、诺马克花岗岩系列（nordmarkite-granite series）、反奥长环斑花岗岩组合（anti-rapakivi associations）以及基性岩。此外还发育有同深成期至晚深成期的霞石岩（nephelinites）、响岩（phonolites）、粗面岩（trachytes）与流纹岩（rhyolites）岩脉。