Alex+-+Newfoundland+Gabbro


 * Newfoundland Gabbro **

Bay of Islands, Newfoundland (See zoomed graphic below)
 * Geographic Origin:**

The sample was most likely collected from Gros Morne National Park in the Bay of Islands region. The park is the second largest park in canada and was established in 1973 for its amazing geologic features.

//Figure 1: O//n the left, //political map of Newfoundland and Labrador, Canada. On the right, satellite imagery of the Bay of Islands, Newfoundland where the sample was collected. The gabbro was collected from the tablelands region, shown in grey on the satellite imagery.//

The area is part of the Long Range Mountains, originally formed 1.2 billion years ago as an outlaying range of the Appalachian Mountains. Unique to the region though is the ophiolite exposure which formed as newly cooled oceanic crust and lower mantle rock (including the moho) was thrusted up and over the continental margin. This is one of the few areas in the world that this feature can be seen and directly studied.
 * Geology of the Bay of Islands:**

Essentially, ophiolites are believed to be large portions of oceanic crust and mantle material that have been brought up onto continental crust. In the Bay of Islands, it is thought that a large collision process uplifted the ophiolite onto the region. Ophiolites are characterized by ultramafic and mafic rock compositions which are indicative of the dense mantle rock versus the surrounding felsic and intermediate continental rock. According to the info page released by Parks Canada, the ophiolite sequence from bottom to top is peridotite, gabbro, sheeted dykes, and finally pillow basalts.

A grey, fairly dense, phaneritic and granular textured igneous rock with various darker and lighter colored portions. Most crystals are fine to medium grained and isotropic in orientation. In hand sample, this gabbro looks more intermediate in composition though this is clearly not the case when mineral composition is determined in thin section. Under the hand lens, crystals of plagioclase feldspar are visible and represent the lighter colored portions.
 * Handsample:**

//Figure 2: Photo of gabbro handsample.//

//Figure 3: Another photo of the gabbro handsample.//

In thin section, this gabbro is a mafic, phaneritic textured, igneous rock. The majority of the sample is isotropic though some fine grained minerals show anisotropic tendencies. The prominent crystals are subhedral to anhedral which provides an initial indication that this gabbro has undergone additional formational processes. These crystals are predominately of two types; orthoclase pyroxene - Augite, which can be easily identified by lines of cleavage across the crystal plain and the distinctive exsolution lamellae and plagioclase feldspar - Albite, which can be identified by twinning and low birefringence. These two minerals in well defined crystals compose roughly 60% of the Gabbro.
 * Thin Section:

The second clue that this rock had undergone alteration processes is the lack of well defined Olivine crystals. In an ideal cooling situation, large well defined Olivine crystals would be formed and easily visible in thin section but in this sample, the olivine is not easily found. Looking at the crystals shown in Figure 7, a fine grained alteration mass can be seen that seems to be composed of two distinct minerals. The first, showing very low birefringence and an alligator/platy texture is probably a variation of serpentine. The second shows similar texture but a much higher birefringence is probably the remaining olivine. The interweaved nature and location of the mass are evidence for alteration. This mass is approximately 30%-35% of the sample.

The remaining composition is a minor amount of opaques and other alteration minerals like chlorite. ** //Figure 4:// //Figure 5:// Figure 6: (Nice thin section pics!!! - ES) Figure 7


 * Interpretation:**