Assignment 7_hand samples etc

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University of Cincinnati, Main Campus *

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3003C

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Geology

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Oct 30, 2023

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docx

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Petrology 3003C, 23SS Assignment 7 Your name(s) here: ___________________________________________________________ The goal of this assignment is to (1) have everyone look at a variety of plutonic and volcanic rocks in hand specimen and practice recognizing various common types of igneous rocks, (2) begin to develop skills in correctly classifying common igneous rocks, (3) look at granite in thin section, and (4) see what a detailed geologic map of a dormant volcanic complex looks like. You can work in pairs or trios, or independently, of course. Part 1. There are three trays of subduction-related volcanics from Japan and Peru. We will call extrusive rocks that are composed of fragments, grains, and/or clasts of various sizes pyroclastic (pyroclasitic rocks “look like clastic sedimentary rocks”). Extrusive rocks composed of minerals and groundmass crystallized from magma will be called volcanic (these rocks look “crystalline”, even if they are aphanitic). 1. Select any two rocks which also have a thin section and describe the hand sample and the thin section. For your thin section description, use the descriptions of J-13 (welded tuff) and K- 41 (augite andesite) as a guide. Part 2. There is a box with three hand samples of granite, each with a thin section: #1, CSI, and BAR. 2. Choose one of these granites and determine their mineralogy, the modes of the major rock- forming minerals, and classify your granite according to its mineralogy (for example, “a biotite granite”). Part 3 In a box labeled 30, there are two igneous rocks with cross cutting relations -- D57c and OQ8. 3a. Sample D57c has a mafic portion with xenoliths of granite and a felsic part with phenocrysts of plagioclase. Which is older, the mafic part or the felsic part? Explain your answer. 3b. Sample OQ8 is composed of a granitic pegmatite and a tan-colored granite. Who is cross cutting who? What is your key observation leading to your conclusion?

Part 4. There is a box with samples of orbicular granite and a box with two samples of the Meredith porphyritic granite. The formation of orbicular granites and feldspar megacrysts is still debated. 4. Pick one of the two granites: orbicular granite or the Meredith porphyritic granite. Research the formation of orbicular granite or feldspar megacrysts and write a one paragraph description of one idea of how they form. Part 5. 5. There are 6 plutonic rocks set out. Using the plutonic rock description template describe and name each rock. Part 6. Find the geologic map titled “Reconnaissance Geologic Map of the West Half of the Crescent 1  by 2  Quadrangle, Central Oregon”. Using the map, answer the following questions: 6a. Find the chart of the Correlation of Map Units. How are the map units organized? 6b. What is the age range of the volcanic rocks? 6c. In the description of the map units, find the units that belong to the Cascade Range . Name three major rock types of this set. 6d. Find the Young Obsidian Flows (Qyo) on the map and in the description of units. Where do these flows occur? What is their age?

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