Texture and Composition of Garnet as a Guide to Contamination of Granitoid Plutons: an Example from the Governor Lake Area, Meguma Terrane, Nova Scotia
Texture and Composition of Garnet as a Guide to Contamination of Granitoid Plutons: an Example from the Governor Lake Area, Meguma Terrane, Nova Scotia
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DOI：
10.3749/canmin.49.2.441
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发表时间：
2011-04
影响因子：
0.9
通讯作者：
K. Scallion;R. Jamieson;S. Barr;C. White;S. Erdmann
中科院分区：
文献类型：
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作者：
K. Scallion;R. Jamieson;S. Barr;C. White;S. Erdmann
新斯科舍省的总督湖地区，主要是由约。373 Ma特拉法尔加深成岩套的花岗岩类岩石侵入了戈登维尔群和哈利法克斯群的变质沉积岩。富石榴石的金维尔群比弗班克组及其包体中的花岗岩类岩石。石榴石单晶，局部直径达3厘米，也存在于双子湖花岗闪长岩和沼泽岛湖英云闪长岩。根据岩相学检验和电子探针数据，我们确定了四种不同类型的石榴石。类型-1石榴石，通常与角质层捕虏体，形成小spessartine丰富（Sps 19-74）集中在不规则的平面聚集颗粒。晶体范围从均匀（1A型）到向Mn富集边缘（1B型）分区。2型石榴石仅限于变质沉积岩的寄主岩石，在那里它形成小的，富含spessartine（Sps 22-71）的颗粒，从富锰的核心地带到贫锰的边缘。3型石榴石含有丰富的变质包裹体（如硅线石），通常具有富含包裹体的核和贫包裹体的边缘。3A型晶体有一个富Mn的核心（Sps 21-52），而3B型晶体有一个富Mn的边缘（Sps 15-21）。4型石榴石形成大的、半自形到自形晶体，具有可能为火成岩成因的丰富内含物（例如磷灰石、斜长石）。4A型石榴石是贫锰尖晶石（Sps 4-11）和弱环带，而4 B型晶体中的环带表明，贫Mn核（Sps 8-16）被部分吸收，并被富Mn边缘（Sps 11-26）过度生长。在花岗岩样品中鉴定出的各种类型的石榴石中，1A和3B型被解释为正辉石晶，1B型被解释为副辉石晶，3A型被解释为具有由正岩浆或副辉石晶边缘过度生长的正辉石晶核，4A和4 B型被解释为正岩浆。富锰捕虏晶石榴石的存在表明，双子湖和沼泽岛湖岩体被含锰岩石污染，可能来自比弗班克组。富锰物质的掺入和同化作用也可能导致花岗质岩浆中正岩浆石榴石的结晶。
The Governor Lake area, Meguma terrane, Nova Scotia, is underlain mainly by ca. 373 Ma granitoid rocks of the Trafalgar plutonic suite that intruded metasedimentary rocks of the Goldenville and Halifax groups. Garnet is abundant in the Beaverbank Formation of the Goldenville Group and its enclaves in the granitoid rocks. Single crystals of garnet, locally up to 3 cm in diameter, are also present in the Twin Lakes granodiorite and Bog Island Lake tonalite. On the basis of petrographic examination and electron-microprobe data, we have identified four different types of garnet. Type-1 garnet, generally associated with coticule xenoliths, forms small spessartine-rich (Sps 19–74 ) grains concentrated in irregular to planar aggregates. Crystals range from homogeneous (type 1A) to zoned toward a Mn-enriched rim (type 1B). Type-2 garnet is restricted to metasedimentary host rocks, where it forms small, spessartine-rich (Sps 22–71 ) grains zoned from a Mn-rich core to a Mn-poor rim. Type-3 garnet contains abundant metamorphic inclusions ( e.g. , sillimanite) and commonly has an inclusion-rich core with an inclusion-poor rim. Type-3A crystals have a Mn-rich core (Sps 21–52 ), whereas type-3B crystals have a Mn-rich rim (Sps 15–21 ). Type-4 garnet forms large, subhedral to euhedral crystals with abundant inclusions ( e.g. , apatite, plagioclase) of probable igneous origin. Type-4A garnet is spessartine-poor (Sps 4–11 ) and weakly zoned, whereas zoning in type-4B crystals suggests that a Mn-poor core (Sps 8–16 ) was partially resorbed and overgrown by a Mn-rich rim (Sps 11–26 ). Of the various types of garnet identified in the granitic samples, types 1A and 3B are interpreted as orthoxenocrysts, type 1B, as paraxenocrysts, type 3A, as having an orthoxenocrystic core overgrown by an orthomagmatic or paraxenocrystic rim, and types 4A and 4B, as orthomagmatic. The presence of Mn-rich xenocrystic garnet suggests that the Twin Lakes and Bog Island Lake plutons were contaminated by manganiferous rocks, probably derived from the Beaverbank Formation. Incorporation and assimilation of Mn-rich material may also have led to crystallization of orthomagmatic garnet in the granitic magmas.