Sara+-+Gaspe+Peninsula+amphibolite

 Introduction and Geologic Background  **
 *  Garnet Amphibolite - Gaspe Peninsula, Quebec, Canada

Amphibolites are metamorphic rocks consisting mainly of amphiboles, usually hornblende. They sometimes have accessory minerals such as Garnet, Plagioclase feldspar, and other common metamorphic minerals such as pyroxenes (diopside) and micas. Amphibolites are usually dark colored, dense/heavy, and exhibit weakly foliated lineations or schistose structure due to the tendency of the hornblendes to line up with each other end to end. (See figure 7)

Amphibolites constitute an intermediate grade metamorphic facies (Figure 21) and tend to form at higher temperatures, between 400 and 750 degrees Celsius, and slightly greater pressure ranges than those needed for the Greenschist facies, but less than those needed for granulites. Amphibolites are associated with major mountain building events (Barrovian or Regional Metamorphism) usually when igneous rocks such as basalts or gabbros are metamorphosed through burial and proximity to batholiths. (Figure 2)

  Hand Samples:

[[image:DSC03693.JPG width="298" height="223" align="center" caption="Figure 3: Large red Garnet crystals surrounded by dark greenish-black hornblende laths "]]






 PPL and XPL Thin Sections 
 * The main components are Garnet, Hornblende (an amphibole), and Diopside (commonly found in metamorphosed igneous rocks)
 * Hornblende is biaxial (-) with a large 2V angle and exhibits up to 2nd order colors (max birefringence 0.019-0.026) - Ca2(Mg, Fe, Al)5 (Al, Si)8O22(OH)2
 * Garnet is isotropic, therefore it goes black in XPL - (Fe, Mg, Ca, Mn)3(Al, Fe)2Si3O12
 * Diopside is biaxial (+) with a large 2V angle and exhibits lower 2nd order interference colors (Max = 0.031) - MgCaSi2O6
 * 40x Magnification





























Interpretation

The Gaspe Peninsula is located on the East coast of Canada within the Humber Zone of the Appalachian Mountain Range. This topographic feature, which extends down the east coast of North America, was created by a series of large-scale orogenies, resulting in regional, or Barrovian, metamorphic facies belts within the region. The Gaspe Belt is the largest middle Paleozoic belt in the Canadian Appalachians, and the obduction of the Mont Albert Ophiolite Complex located on the Gaspe Peninsula occurred simultaneously with the obduction of the Bay of Islands Ophiolite Complex in Newfoundland, Canada, although it is observed that the metamorphism on the Gaspe Peninsula was of higher grade than in Newfoundland. [|40Ar/39Ar] dating places this event during the Taconic Orogeny, which began in the Cambrian (550 ma) and lasted until the End Ordovician (440 ma), largely due to the closing of the Iapetus Ocean. Rocks of the Gaspe belt rest unconformably on Cambrian to Ordovician rocks, and are then unconformably overlain by Carboniferous rocks. This suggests that the protolith of this Garnet Amphibolite, which is igneous (mafic) in origin due to the elemental compositions of the remaining constituents, was most likely oceanic crust from the back-arc basin of the spreading center that was obducted onto land as part of the ophiolite complex. The Garnet Amphibolites collected from the Gaspe Peninsula most likely formed as part of the metamorphic contact aureole around the ophiolite itself, which would have produced extreme temperatures and moderate pressures resulting in this intermediate grade metamorphic facies (Figure 21.) Subsequently, only after uplift and erosion has occurred, are these Garnet Amphibolites exposed at the surface. The three main constituents of these Garnet Amphibolites are garnet, Hornblende, and Diopside. The garnets associated with this type of amphibolite are most likely Pryope garnets (Mg3Al2(SiO4)3), but could also be almandine (Fe3Al2(SiO4)3) since there are possible Fe-alteration rims around some of the garnet crystals. Garnet is a mineral easily formed during metamorphism, therefore it is found almost exclusively within metamorphic rocks and can thus be an indicator of such. Diopside is a clinopyroxene mineral that forms a solid solution with Augite and Hedenbergite, and is very common in igneous rocks, such as basalt, and their associated metamorphic counterparts. Usually amphibolites contain abundant Plagioclade Feldspar, although this rock has none. This suggests that whatever Ca and Al present went into the formation of the Garnet and Diopside, with no ingredients left over for the formation of Plagioclase. In other words, the bulk composition and subsequent metamorphic conditions resulted in an Amphibolite with only Garnet, Hornblende, and Diopside. Hornblende itself is a hydrated mineral, thereby indicating that this environment was not anhydrous or dewatered. This mineral assemblage suggests formation pressures of ~1GPa, and formation temperatures between 400-500 ° [|Celsius] (Figure 22.)

The rough compositional breakdown = 70% Hornblende, 20% Garnet, 10% Diopside





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