Porphyroblasts

A porphyroblast is a texture in many metamorphic rocks that have certain minerals that are larger than the rest of the rock matrix in grain size. The large grains are known as porphyroblasts and the rock has a porphyroblastic texture. Many porphyroblasts form in pelitic rocks. Some of these minerals are garnet, staurolite, cordierite, kyanite, sillimanite, and andalusite. In general the higher the metamorphic grade the larger the porphyroblasts can become. Different minerals form at different temperatures and pressures so rocks such as quartzite and marble that only contain one mineral do not form prophyroblasts.

Garnet porphyroblasts in a pelitic rock.

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Volcanic Necks

Volcanic necks form from feeder pipes that were once inside volcanoes, but once the volcano goes extinct and is in an erosional environment, the volcano weathers away but the neck remains. They are intrusive bodies, but near the surface the morphology of the necks can be cylindrical. At depth necks can become elliptical or even clover shaped. At great depths they can merge into several feeder dikes.

Devil’s Tower, Wyoming

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Flood Basalts

Flood basalts usually erupt in zones of crustal extension, this typically occurs along mid ocean ridges but can form on continental crust. Large flood basalts are called Large Ingneous Provinces (LIP). Flood basalts can be as large as millions of square kilometers in area. The flood basalts  form flat layers that mark each single eruptive event. Some ancient flood basalts are the Deccan traps in India that are from the Creataceous and they are believed to have some significance of the K-T extinction event. The Deccan traps are about 500,000 square kilometers in area and about 600 meters thick. The Miocene Columbia River basalts in the state of Washington are about 200,000 kilometers squared and about 1500 meters thick. Another large flood basalt that could have had some affect during the Permian extinction are the Siberian traps in Siberia. They range from 1,000,000 to 4,000,000 cubic kilometers in volumes. the flood basalts are usually dominated by randomly oriented plagioclase, pyroxene, and olivine grains.

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Progressive Metamorphism

The study of progressive metamorphism began in the early 1900’s in the Scottish Highlands. George Barrow was mapping the geology of the area and he realized that there were index minerals that characterized each zone. If he was in the lower grade metamorphic zones ceratin minerals were no longer present. The zones that were closest to the principle stress had the highest grade of metamorphic minerals and the further away from there shows lower metamorphic grade. each mineral zone is separated by a boundary called an isograd. Barrow mapped each zone in the order of going from low grade to high grade starting with the chlorite zone –>biotite –>garnet –> kyanite –>sillimanite. These are now referred to as Barrow’s Zones.

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Metamorphic Facies Concept

metamorphic facies are a group of minerals that form at certain pressure and temperature conditions. For example if the minerals in the rock formed at pressures greater than 1 GPa and 500 degrees C, then this mineral assemblage will fall into the eclogite facies. mineral assemblages that undergo high presures but lower temperatures such as at convergent plate margins are known as the bluescist facies. At even lower temperatures is the greeenschist facies. Some of the minerals that fall into the eclogite facies are garnet and orthopyroxene. Some of the minerals in blueschist facies can be kyanite and gluacophane, and some minerals in the greenschist facies can be chlorite, epidote, and actinolite.

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Volcanic Stratigraphy and Petrology of the Northern Snaefellsnes Rift, Southern Laxardalsfjoll, Iceland

UPJ geology students….We need to raise money to get our asses to Iceland next summer. So start raising some money….even if we have to panhandle.

The volcanic rocks of the southern Laxardalsfjoll range in the northern Snaefellsnes rift are predominantly Tertiary tholeiitic basalt flows. Major and trace element data for these basalts, when compared to typical primary N-MORB, indicate they are evolved.

     

The plagioclase phyric andesitic unit (Strjugsskard andesite) contains up to 35% euhedral plagioclase phenocrysts ranging up to several centimeters in diameter. The groundmass is generally dark gray to purple near the bottom of the flow, becoming more red toward the top.

See figure 2 in text.

http://keckgeology.org/files/pdf/symvol/18th/iceland/schuyler.pdf

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Geology and Petrology of the Pribilof Islands Alaska

This isn’t the creme de la creme of geology papers in my opinion, but i enjoyed it regardless.

Lava flows and sills of basaltic habit make up the bulk of St. Paul island. Some individual flows seem to merge into each other, but in many places they can be distinguished and thicknesses from 10 to 7 yards were measured; still thicker flows probably exists. Peridotite is probably the most interesting rock in St. George island. It is overlain by sills and lava flows.  Aplite is intrusive into the peridotite and composite in grained granular central portion.

Here is the link.  http://response.restoration.noaa.gov/book_shelf/535_prib5.pdf

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Petrology of Jurassic Coal, Hill River Area, Perth Basin, Wester Australia

Basic description of coal written by a grad student getting her doctorate in philosophy.

The coal measures subcrop in a half-graben bounded by the Leseuer-Peron Fault in the west, and the Warrendarge Fault in the east. The coal occurs within the shallow sequence of the Cattamarra member which is also described  as the Cattamara Coal measures of the Cockelshell Gully Formation.

Samples from 6 drill holes were examined for petrological and geochemical investigation. The coal predominantly comprises of banded, dull banded and dull lithotypes, with minor branded , bright and fusainous types.

check this out

Link: http://espace.library.curtin.edu.au:1802/view/action/nmets.do?DOCCHOICE=15765.xml&dvs=1260579094535~396&locale=en_US&search_terms=000013824&usePid1=true&usePid2=true

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Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples

Good paper talking about the  nature of cometary solids  of fundamental importance to the understanding of the early solar nebula and protoplanetary history.  Posted are really nice pictures of Bright-field TEM images of Wild 2 grains and diagrams of ranges of Fe-Ni sulfides from six grains from five Wild 2 particle tracks. 

In summary The presence of a refractory particle resembling a meteoritic CAI among the Wild 2 grains raises many new questions. IDPs are believed to contain samples of both asteroids and comets, and wholly refractory IDPs were identified two decades ago.  The emerging model of the structure of the captured grains is that many were predominantly very fine-grained (submicrometer-sized) loosely bound aggregates with a bulk chondritic composition.

Please read

http://www.sciencemag.org/cgi/content/full/314/5806/1735

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Clay petrology of Cambro-Ordivician Continental Margin, Cow Head Klippe, Wester Newfoundland

I have little knowledge of clay petrology so I decided to take some time out and read this article.

This talks about Cow Head Breccia of middle cambrian to late lower ordovician age and the overlying middle ordovician “red shale”. The clay minerals in the Cow Head are illite, mixed-layered illite-smectite, chlorite, expandable chlorite and corrensite. They occur in three clay layered mineral suites named for the dominant clay minerals.  The Cow Head Klippe in western Newfoundland contains three stratigraphically restricted suites of clay minerals

Beginning in the early lower ordivician , rich volcanic detritus was transported westward from a developing volcanic arc in central Newfoundland. 

http://www.clays.org/journal/archive/volume%2025/25-3-163.pdf

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