Download GEOL 2312 – Igneous and Metamorphic Petrology

GEOL 2312 – Igneous and Metamorphic Petrology Spring 2016 Name__________Key_________ Score _____/ 53 MIDTERM EXAM II 1) Name three processes or conditions that occur in the mantle that will yield variable magma compositions. (1.5 pts) Changing pressure/depth changing volatile content, variable degrees of partial melting, heterogenous mantle composition fractional crystallization during ascent through the mantle 2) Name two specific conditions of mantle melting that will favor the creation of alkaline basalt magma over a tholeiitic composition. (1 pt) Low degrees of partial melting, deep melting, CO2 enriched mantle 3) Fractional crystallization driving magmatic differentiation is a common way to create magma diversity in crustal settings. (1 pt) A) What rate of cooling is necessary for fractional crystallization to occur? Slow cooling B) What physically must happen between crystals and magma for differentiation to occur? Must physically separate from each other 4) When we look at cumulate rocks in a mafic layered intrusion, what mineral habit identifies the minerals that fractionally crystallized from the magma chamber? (Circle one; 0.5 pt) 1) Subpoikilitic B. Subhedral granular C. Poikilitic 5) When a mafic magma is saturated in olivine, plagioclase, augite and Fe oxide, what is the ideal (eutectic) proportions of these four cumulus minerals? (Circle one; 0.5 pt) Ol:Pl:Cpx:FeOx = 30:20:30:20 5:30:30:35 10:55:25:10 50:20:20:10 6) Listed in the table below are the compositions of a mafic magma and of olivine and plagioclase which would be in equilibrium with that magma. Assuming those minerals crystallize in the cotectic proportions of 35% Ol and 65% Pl, 1) calculate the composition of the total solid for each elemental component. If this troctolitic assemblage fractionally crystallizes from the parent magma, then 2) indicate whether each elemental component will decrease or increase in abundance (circle – or +) in the differentiated magma. (4 pts) Elements Parent Magma 35% Ol 65%Pl Total Solid Dec or Inc? SiO2 48.0 35.0 52.0 46 ‐ / + TiO2 2.0 ‐‐‐ ‐‐‐ 0 ‐ / + Al2O3 15 ‐‐‐ 30.0 19 ‐ / + FeO 14 20.0 ‐‐‐‐ 7 ‐ / + MgO 9.0 45.0 ‐‐‐‐ 16 ‐ / + CaO 9.0 ‐‐‐ 15.0 10 ‐ / + Na2O 2.3 ‐‐‐ 2.7 1.8 ‐ / + K2O 0.5 ‐‐‐ 0.3 0.2 ‐ / + 7) For the same fractionating assemblage as in #7 and trace element mineral partition coefficients listed below for Ni, Sr, La and Zr, in olivine and plagioclase, indicate whether fractional crystallization will result in these elements becoming depleted (‐), enriched (+), or strongly enriched (++) (2 pts) Kd Ol Kd Pl (‐) / (+) / (++) Ni 14 .001 ‐ Sr .005 1.5 ‐ or + La .001 .1 + or ++ Zr .005 .008 ++ 8) Besides magmatic differentiation, name other two processes that occur in the crust which can further create diverse magma compositions (1 pt) Magma mixing crustal melting crustal assimilation 9) Indicate in the columns to the right, what stratigraphic progression of cumulus mineral assemblages will be created in a mafic layered intrusion by the fractional crystallization of parent magmas A and B; use the abbreviations: Ol‐olivine, Pl‐Plagioclase, and Px‐Pyroxene (2 pts) Magma A Magma B
Cpx+ Pl + Ol Pl + Ol Pl Px+ Pl + Ol Px + Ol Ol 10) The photo below shows the common phenomenon of incomplete mixing of mafic and felsic magmas. Why does this occur? (0.5 pt) Viscosity contrast between mafic and felsic magmas 11) The igneous stratigraphy of the classic Skaergaard intrusion is shown in the figure on the next page. Label the arrival (or disappearance) of the cumulus mineral that defines the various zones of the intrusion from the list of cumulus minerals to the right (2.5 pts). +Fe‐amphibole + Apatite +Fe‐olivine + Apatite + Augite + Fe‐Ti oxide ‐ Olivine + Fe‐amphibole + Fe‐olivine ‐ Olivine ___+Fe‐Ti oxide ___+Augite __Olivine + Plagioclase 12) In the two phase diagrams below, two different parent magma compositions are plotted. One represent magmas that would give rise to tholeiitic mafic intrusions like Skaergaard and Sonju Lake, the other would give rise to ultramafic mafic intrusions like the Bushveld Complex. Label the likely Skaergaard parent magma with an “S” and the likely Bushveld magma with a “B” (2 pts) S
S B
B 13) The Cyclic Zone of the Layered Series at Duluth shows multiple cycles where Ol+Pl cumulates grade up into Ol+Pl+Pyx+FeOx cumulates and then abruptly regress back to Ol+Pl cumulates at the base of the next cycle. Describe two processes that might explain this cyclicity and what evidence you would look for to support this (2 pts). Process 1: magma recharge Evidence: increase in mg# at cycle boundaries Process 2: magma venting Evidence: change in phase assemblage, abrupt decrease in grain size 14) The main components of a typical ophiolite sequence are listed below. Put these terms in the proper stratigraphic order from top to bottom (2 pts) Terms: Sediments, Gabbros, Ultramafic Rocks, Sheeted Dikes, Pillowed Basalt Top Sediments ‐‐> pillowed basalts‐‐>sheeted dikes‐‐> Gabbros‐‐>ultramafic rocks Bottom 15) Indicate with an F or an S whether the statements below apply to fast‐spreading or slow spreading ocean ridges. (2 pts) F Spreading rates fall in the range of 3‐6 cm/year S Marked by a 30‐50 km‐wide axial valley and 3‐9 km inner valley F Marked by a narrow (2‐5 km) axial ridge S Volcanism occurs in isolated eruption centers F Volcanism occurs semi‐continuously along fissures F Generally contains more fractionated basalts (lower mg#) F Creates more sheet basalt flows as opposed to pillow basalts S Associated with shorter magmatic and tectonic segments 16) For the REE normalization plots of E‐MORB and N‐MORB, answer whether the questions below as ‐
MORB, N‐MORB, neither, or both. (2 pts) A) From a depleted mantle source N‐MORB B) From an enriched mantle source E‐MORB C) From a garnet‐free mantle source Both D) From a garnet‐bearing mantle source Neither 17) The diagram below shows the development of a Hawaiian Island as the Pacific plate drifts over a mantle plume. Label the occurrences of normal mid‐ocean ridge basalt (NMORB), alkaline ocean island basalt (AOIB), and tholeiitic ocean island basalt (TOIB) produced from such a process. (2 pts) AIOB
AIOB
TOIB
N‐MORB 18) Although continental flood basalts are rare in the Precambrian, what is the main evidence for their prior existence? (0.5 pt) Dike Swarms 19) How would you explain the greater volume of basalts comprising the Columbia River Group (CRBG) compared to the Snake River Plain (SRP)? (0.5 pt) CRBG fed by mantle plume head; SRP fed by plume tail 20) Name two pieces of evidence for the existence of a significant mafic underplate beneath the Midcontinent Rift in the Lake Superior region (1 pt). Geophysics (gravity), felsic magmas,
anorthosites
20) Give one piece of evidence that suggests to most petrologists that the Midcontinent Rift was created by a mantle plume (0.5 pt) Large volume of mafic magma, Evidence of high mantle T, geochemisty
21) Why must we suffer having Wisconsin as a neighbor (and a sports rival), when by all rights it should be part of a separate continent? (0.5 pt) Grenvillian compression 22) Both tholeiitic and calc‐alkaline compositions occur in island arc systems. On the AFM diagram below, label the compositional fields that define these two types of subalkaline magmas. (0.5 pt) Tholeiitic
Calc‐Alkaline
23) In the diagram below of a typical island arc, label the tectonomagmatic features with the appropriate letters from the list to the right. (2pts) D C B
E
A. Mantle Wedge B. Volcanic Arc C. Forearc D. Trench E. Backarc A
24) What minor element is commonly used to discriminate different magma series in island arc systems (0.5 pt) K, potassium 25) In the trace element spider diagram comparing trace element abundances in basalts from island arcs and OIBs relative to MORB….(2.5 pts) A) Name 3 elements that are similar in abundance between IA and MORB? Nb Zr Hf Sm Ti Y Yb_ B) Name a LIL element that is similar between IA and OIB Sr K Rb Ba C) Name a HFSE element that is similar in abundance between IA and OIB Y Yb (Ce) D) What element is anomalously low in abundance in IA basalts relative to comparably incompatible trace elements? Nb 26) Name three factors that control the extent to which the geotherms will be depressed due to subduction of oceanic lithosphere as shown in the figure below (1.5 pts) Rate of Subduction Age of the Oceanic Lithosphere Age of the Volcanic Arc Extent to which the subduction induces flow of the mantle wedge 27) In the diagram to the right, the numbers 1‐5 show possible source areas for generating magmas in island arc systems. Which source is likely produce the most magma to the arc?; 2 Which is likely to produce the least? 4 or 5 (1 pt) 28) The diagram to the right shows pressure‐
temperature paths of subducting oceanic lithosphere for different ages of the arc and different ages of the subducting lithosphere. Also shown are the wet and dry solidus curves for basaltic crust. Based on these relationships… (1.5 pts) A. What age conditions would lead to direct melting of the subducting slab? Very young, < 5Ma B. What role do chlorite and hornblende play in promoting melting of the slab? Their dehydration melting make water available to lower the melting point of the mantle_ C. Given that chlorite and hornblende are not common igneous minerals in a basalt, how do these minerals form in ocean crust? Hydrothermal alteration of basaltic crust at mid‐ocean ridges transform igneous minerals into hydrothermal mineral such as chlorite and hornblende 29) In a mature arc system, as shown in the diagram below (left), melting of the mantle wedge typically occurs at levels A and B. Referring to the phase relationships shown in the adjacent figure, describe the process occurring at depths of 75, 125 and 200 kilometers that promotes this melting (1.5 pts) 75 Km – dehydration of the ocean crust puts water into the mantle wedge 125 Km – dehydration melting of paragasite (amph) triggers melting 200 Km – dehydration melting of phlogopite (biotite) triggers melting 30) What is one possible explanation for the gaps in volcanic activity along the Andean arc? (0.5 pt) Low angle of subduction or over‐thickened ocean crust 31) Compared to island arcs, basalt and basaltic andesite are much less common volcanic products in continental arcs. What is a likely explanation for this? (0.5 pt) Mafic magmas have a harder time traversing thick continental arcs 32) Continental arcs are notoriously diverse in their magma compositions. One process that commonly produces this diversity is by the ponding of mantle melts at the base of the crust, which triggers the “MASH” process. What does this acronym stand for? (At least describe the process in your own words) (0.5 pt) Melting Assimilation Storage Homogenization 33) A long‐standing controversy existed over whether granites are produced largely by extreme fractional crystallization or by partial melting of crust. Why do most petrologists believe that partial melting is responsible for the formation of most felsic magma? (0.5 pt). Differentiation of mafic magmas by fractional crystallization can produce only a small volume (~5%) of felsic melt 34) True or False about Continental Arc Plutonic Complexes (or batholiths) (3 pts) T F Continental arc plutonic complexes are the exposed fossil magma chambers of continental arc volcanics T F Unlike gabbroic rocks of mafic layered intrusions, the compositions of most plutonic rocks comprising continental arc batholiths are similar to their parent magma compositions. T F Batholiths are typically composite intrusions of many different intermediate to felsic rock types. T F Over time, continental arc plutonic complexes grade from felsic compositions early to more intermediate compositions late T F Rocks comprising continental arc batholiths rarely show signs of crustal melting or contamination. T F The Sierra Nevada batholith formed when margin of the North American plate off California (now a transform boundary) was an active rift zone. 35) Match the dominant source material that is thought to give rise to the four different SIAM granite types. (2 pts) S type: A A. Partial melting of sedimentary rocks I type: D B. Partial melting of lower crustal rocks heated a mantle plume A type: B C. Extreme fractional crystallization of mafic magmas M type: C D. Remelting of hydrous gabbroic rocks in the lower crust 36) On the illustration below of a continent‐continent collision zone, circle the area in the crust where felsic magmas might be expected to be generated (0.5 pt). .5
37) True or False about Continent Alkaline Magmatism (2 pts) T F Strongly alkaline rocks probably involve some degree of enrichment of alkalis and other incompatible elements by metasomatic fluids. T F Alkaline rock in continental setting show a more limited range of Na/K ratios than their oceanic counterparts. T F Alkaline magmas contain so much Na and K that they use up all the Si to make feldspars and feldspathoids with none left to make quartz. T F Trace element and isotopic data show that most highly alkaline rocks have a strong crustal component 38) The presence of diamonds in kimberlites indicates what about their source and their emplacement history in the crust? (1 pt) Source: Deep mantle source (> 150 km) Emplacement: Rapid ascent 39) What is the “Anorthosite Problem”? (0.5 pt) No known magma compositions that could create anorthositic rocks. 40) State two lines of evidence that anorthositic rocks of the Duluth Complex were derived from plagioclase crystal mushes tapped from lower crustal magma chambers (1 pt) Plagioclase zoning patterns, complex internal structure (foliation), no evidence for ultramafic rocks in upper crust, plagioclase should float under lower crustal pressures, anorthositic rocks don’t grade into normal gabbros, …