GEOL 2312 Igneous and Metamorphic Petrology Spring 2009 Sc ore / 40

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1 GEOL 2312 Igneous and Metamorphic Petrology Name Spring 2009 Sc ore / 40 QUIZ 3 1) Name two geologic features that provide physical evidence for the mineralogy of the earth s mantle (2 pts) Ophiolites, Kimberlites, Mantle inclusions in basalts, Dredge samples from ocean fracture zones 2) In addition to olivine, clinopyroxene and orthopyroxene, the mantle is composed of Al bearing phases of either plagioclase, garnet or spinel, depending on depth/pressure. Which of these is the low pressure phase? Plagioclase; which is the high pressure phase? Garnet 1pt) 3) Under normal geothermal gradient conditions, the mantle will not melt. Name two processes or conditions that allows the mantle to melt (2 pts) Increasing T (mantle plumes), Adiabatic decompression, Increasing volatiles, 4) Name three processes or conditions that occur in the mantle that will yield variable magma compositions. (3 pts) Changing pressure/depth, changing volatile pressure, variable degrees of partial melting, fractional crystallization during ascent through the mantle, heterogenous mantle composition 5) Name two specific conditions of mantle melting that will favor the creation of alkaline basalt magma over a tholeiitic composition. (2 pts) Low degrees of partial melting, deep melting, CO 2 enriched mantle 6) Fractional crystallization driving magmatic differentiation is a common way to create magma diversity in crustal settings. Name two conditions are that are necessary for fractional crystallization to occur. (2 pts) Slow cooling, different densities between crystals and magma 7) When we look at cumulate rocks in a mafic layered intrusion, what textural attribute do we look for to determine what minerals fractionally crystallized from the magma chamber? (1 pt) Minerals that have granular habits approaching their free growth forms 8) 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. If those minerals fractionally crystallized in the cotectic proportions of 35% Ol and 65% Pl, circle the elemental components that should increase in abundance in the differentiated magma. (3 pts)

2 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 Ol+Px+Pl Pl+Ol+Px Ol + Px Pl + Ol Ol Pl 10) Besides magmatic differentiation, name other two processes that occur in the crust which can further create diverse magma compositions (2 pts) Magma mixing, crustal melting, crustal assimilation 11) The photo below shows the common phenomenon of incomplete mixing of mafic and felsic magma. Why does this occur? (1 pt) Strongly contrasting viscosity makes magmas behave like immiscible liquids; also mafic magma will quench in the presence of lower temperature felsic magma

3 12) The igneous stratigraphy of the classic Skaergaard intrusion is shown in the figure below. 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 (3 pts). + Fe amphibole + Apaite + Fe olivine + Apatite + Augite + Fe Ti oxide Olivine + Fe amphibole + Fe olivine Olivine + Fe Ti oxide + Augite Olivine + Plagioclase 13) There are two very different schools of thought about how mafic layered intrusions crystallize. One, touted by Wager and more recently by Irvine can be termed the dynamic magmatic sedimentation model. The other championed by McBirney and others is often termed the static boundary layer model. Some of the phenomenon of mafic layered intrusions for which the two schools have different interpretations include the physical process driving magmatic differentiation, magma convention, liquid line of descent, origin of igneous foliation (or lamination), origin of modal layering, origin of trough layering, and the origin of anorthositic blocks. Choose one of these phenomenon and briefly describe the different interpretations of the two schools. (2 pts) Phenomenon

4 Dynamic Magmatic Sedimentation Static Boundary Layer 14) 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) B S S B 15) 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/Fe in Ol & Pyx, Ca/Na in Pl Process 2: Decompression under water saturated conditions Evidence: No change in mineral chemistry, hydrous minerals, quench textures 16) The main components of a typical ophiolite sequence are listed below. Put these terms in the proper 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

5 17) What does the enrichment of Light Rare Earth Elements (LREE) in EMORB and the depletion of LREE in NMORB tell us about the mantle sources of these two basalt compositions? (1 pt) EMORB came from undepletedd (fertile) mantle while NMORB was derived from previously melted and depleted mantle 18) 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) producedd from such a process. (2 pts) Alkali Basalt Tholeiitic Basalt NMORB 19) Although continental flood basalts are rare in the Precambrian, what is the main evidence for their prior existence? (1 pt) Dike Swarms

6 20) How would you explain the greater volume of basalts comprising the Columbia River basalts compared to the Snake River Plain lavas? (1 pt) CRB generated from the large head of a starting plume; SRP generated from plume tail 21) Name two pieces of evidence for the existence of a significant mafic underplate beneath the Midcontinent Rift in the Lake Superior region (2 pt). Gravity modeling, Anorthositic rocks, Felsic magmas from melting the lower crust, Overthickened crust 22) 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? (1 pt) Grenville orogeny caused compression of the rift after the plume energy was expended Extra Credit 1) What type of volcanism occurred during the latent magmatic stage of the Midcontinent Rift? (1 pt) Felsic only 2) Ocean island basalts are thought to be generated from what feature of the mantle? (1 pt) mantle plumes 3) Why is the East Pacific Rise a fast spreading ridge? (1 pt) The Pacific is ringed by subduction zones where slab pull tugs at the oceanic crust; also, not pushing continental crust around 4) Describe a physical feature of the Skaergaard intrusion that support the magmatic sedimentation model (1 pt). Cross bedded layering, graded layering, drape layering over inclusions, trough layering