mid term exam - Petrology 2023_individual

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Geology

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Oct 30, 2023

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Petrology, GEOL 3003C, spring 2023, mid-term exam. Your name here: _____Olivia Girten______ Please type your answers in the Word doc. Use a much space as you wish. You can answer the multiple-choice questions by highlighting your answer(s). Please upload your completed exam into our Canvas course – in the module “mid-term exam” in the Individual mid-term assignment. Part 1. HAND SAMPLES – “ROCKS IN BOXES” – TO COMPLETE THIS PART OF THE EXAM, EXAMINE THE HAND SAMPLES WHICH ARE ARRANGED IN 5 BOXES ON THE BACK BENCH IN ROOM 527 . Please do not mix up the box labels…or the samples! [4 points for each box unless otherwise noted.] Find Box 1 – Are these igneous rocks volcanic or plutonic? plutonic Consider them as a group. What name would you give them that describes their overall bulk chemistry? What is/are one or two specific rock names would you give them? Gabbro/mafic, phaneritic texture dark colored, some look like they have olivine. What might be the tectonic setting for these rocks? [ Choose one ] a. continental breakup b. oceanic hot spot c. oceanic magmatic arc d. none of the above Find Box 2 – All of these rocks are examples of the Devonian Kinsman quartz monzonite* from central New Hampshire. Briefly describe the prominent texture of these rocks (which are essentially the same, sample to sample). Some of these samples show a preferred shape orientation of the minerals and are foliated. Are these fabrics a primary igneous fabric (perhaps an “igneous flow fabric”) or did they form during deformation (that is, they are “tectonic fabrics”)? Briefly explain your reasoning. Pegmatic texture, very large grains really beautiful rocks, I believe that the reasoning behind the preferred orientation is deformation. The grains that do not look like they are in a preferred orientation, look as though they were deposited after, intruding the foliated minerals which is a norm for deformation. *monzonite – plutonic igneous rock rich in feldspar and low in quartz, plagioclase > alkali feldspar, mode of light minerals is 55-90% of which 80-100% are feldspar (plagioclase 35-65%, alkali feldspar 35-65%),

20-0% quartz; accessory minerals include pyroxene, hornblende, biotite; transitions to gabbro in which plagioclase is more dominant, quartz is < 5%, and pyroxene up to 20%. Find Box 3 – careful, these samples are a bit messy… Are these rocks volcanic or plutonic? volcanic Consider them as a group. What name would you give them that describes their overall bulk chemistry? What is a specific rock name might you give them? I would say these are pumice rocks, they have a porphyritic texture and felsic. What is a likely tectonic setting for these rocks? [ Choose one ] a. hot spot volcano b. MOR c. supra-subduction zone d. supervolcano Find Box 4 – In this box, there are four metamorphic rocks. Each rock has a label on it, but also a number in its little box. Here are the rocks: Box 1 --sample label VT1 Box 2 – sample label MC.90.3 Box 3 – sample label OC.89.5.1C Box 4 – sample label M6A Taken together – and pretending that all four rocks are geologically related to one another – in what tectonic setting were they metamorphosed? The tectonic setting they were metamorphosed in was an orogenic subduction zone deposit. Pick one of the four and in a short paragraph, write up what it tells you. Box 2 – sample label MC.90.3 This rock appears to be a basalt, which would make it volcanic, aphanitic texture which tells me it cooled rather quickly which only confirms to me that it is a this is also an extrusive rock. The texture is

also relativiely uniform, almost no nucleation can be seem with the naked eye. All of these things confirm that its volcanic, most likely felsic, extrusive, and that it cooled quickly. Find Box 5 – These rocks are samples of the Ammonoosuc Volcanics, collected in central Massachusetts and southern New Hampshire. These rocks could all be reasonably called _________ metamorphosed basalt_________ [use a rock name] If – based on your looking these rocks and thinking about them a bit -- you think they have been metamorphosed, what metamorphic facies do they represent? The blueschist facies Name three minerals you would expect to find in these rocks. Glaucophane, lawsonite, and epidote Part 2. Answer the following questions: Study the figure below and use it answer Questions 1a, 1b, 1c, 1d, and 1e below. The horizontal lines are the boundaries between the lithosphere and asthenosphere, and the asthenosphere and the upper mantle (the asthenosphere isn’t labeled). The cusps in the Anhydrous solidus reflect changes in the Al- bearing mineral in the mantle (at about 25 km, plagioclase changing to Al-bearing spinel, and at about 60 km, spinel changing to garnet).

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1a. We have a mantle rock located at point C, in the asthenosphere, along the geotherm. Consider this rock to be completely dry, that is, it is anhydrous. We are interested in melting this rock. Consider path A: this path describes…[ choose all correct answers ] [2 pts] a. addition of heat b. isothermal heating c. isobaric heating d. addition of water e. perturbing the geotherm 1b. At about what temperature does our mantle rock partially melt if it follows path A? (Feel free to draw on the diagram.) [2 pts.] Our mantle rock would partially melt somewhere right below the total liquidus line, because above it everything is completely liquid. 1c. Consider path B, labelled “Adiabat”. What does this path describe? [ choose one ] [1 pt.] a. nearly isobaric cooling b. nearly isothermal decompression c. addition of water d. changing the geotherm by losing heat 1d. At about what temperature does our mantle rock partially melt if it follows path B? Upper Mantle

Given if our rock follows path be, it will partially melt around 1250 degrees 1e. Path B might best be called…[1 pt.] a. exhumation melting b. adiabatic melting c. slow cooling melting d. decompressional melting 2. Name two ways in which are olivine and diopside different? Name two ways in which they are similar. [4 pts.] The crystal fractionation of olivine and diopside are different, and both look different in xpl They are both similar because they have little to no cleavage and they both have the same green color in ppl. 3. Why do some volcanoes erupt explosively, while others do not? [2 pts.] All volcanoes are different, some erupt explosively due to the pressure that is built up in the magma chamber below and it also depends on the silica content of the magma as well as other compositional aspects of the magma.The silica content determines the viscosity of the magma. You also have to consider the timeframe of eruption(s), how long has this volcano last erupted? 4. In the space below, draw a schematic P-T path (with P on the Y-axis, T on the X-axis) that follows this sequence of events for a sedimentary rock which is first located at the bottom of a 5,000-meter thick basin: (1) instantaneous overthrusting by a 10 km thick thrust sheet; (2) heating for 10 million years with zero exhumation; (3) exhumation of this rock to Earth’s surface. Label each segment. [5pts.] have mercy I did my best on this one.

5. Use the facies diagram below to answer questions 5a, 5b, and 5c. 5a. Consider the burial path described by the white arrow. What happens to the water content of a meta-basalt that follows this path? What might be the tectonic setting of this path? [3 pts.] The water content of the meta-basalt that follows this path decreases as it goes from blueschist, amphibole eclogite and finally diamond eclogite. The tectonic setting of this path might be a subducting plate going farther and farther in the mantle. Since blueschist and eclogite is involved I would go with anorogenic subduction zone. 5b. Consider the low-pressure path described by the black arrow; this a path in the upper crust. How might this path be created? [1 pt.] a. by overthrusting at a convergent plate boundary b. by hydrothermal circulation c. by intrusion of a mafic or ultramafic magma into the shallow crust d. by intrusion of a granitic magma into the shallow crust

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5c. What are the boundaries between the different facies? [1 pt.] a. the locations of specific mineral reactions that change assemblages b. lines determined by field studies c. Barrow’s original facies isograds d. aesthetically pleasing, but arbitrary lines 6. Increasing mineral grain size typically occurs as rocks are metamorphosed to higher temperature. Why? [1 pt.] a. recrystallization becomes more important than deformation as grade increases. b. increasing grain size reduces the high energy surface areas of minerals c. the minerals that crystallize at high temperature are, in general, larger than minerals that crystallize at low temperature d. deformation – which reduces grain size – is not as strong at high temperature as it is at low temperature 7. In the shallow crust, diagenetic (that is, sedimentary) processes transition into metamorphic processes. How do metamorphic processes transition into igneous processes in the deep crust? [2 pts.] Metamorphic processes transition into igneous processes in the deep crust by fully melting and rechanging the composition via crystallization and then eventually being exhumated back up through the mantle.