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Trace-Element Modeling of the Petrogenesis of Granophyres And

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Trace-Element Modeling of the Petrogenesis of Granophyres And American Mineralogist, Volume 71, pages 460-47 I, 1986 Trace-elementmodeling of the petrogenesisof granophyresand aplites in the Notch Peak granitic stockoUtah Pnrnn I. Nlrelr. Department of Geology, University of Missouri-Columbia, Columbia, Missouri 6521I AssrRAcr The petrogenesesof a granophyre and an aplite in the Notch Peak stock, lJtah, were modeled using published experimentaldata for mineral/melt, mineraVfluid, and melVfluid partitioning of alkali, alkaline-earth and rare-earth elements.The stock rangesin compo- sition from granite to quartz monzonite. The granophyreoccurs along a horizontal contact of the granite with overlying impermeable limestones and contains graphic intergrowths ofalkali feldsparand quartz, displayingunidirectional solidification structures.Calculations suggestthat the granophyre formed by 2-l3o/o crystallization from an aqueousfluid that exsolved from the magma during emplacementand was trapped beneath the limestones. The observed strong enrichment of Ba and Sr in the granophyre relative to the ganite is due to largealkali feldspar/fluid partition coefficientsfor theseelements, while the depletion of large-ionJithophile elementsis due to their small solubility in aqueousfluids. On the other hand, the aplite, one of many crosscuttingthe stock, is depleted in Ba, Sr, and the REEs, while the concentration of highly chargedelements, Zr,Ta, Hf, Th, and IJ, is about the same as in the major lithologic units in the stock. It is not possible to explain the alkaline-earth element and REE concentrationsin the aplite by any model that employs mineral/melt partition coefrcients for anhydrousmelts. However, the "minimum granite" composition and the large concentration of the highly charged elements are difficult to explain by direct crystallization from aqueous fluids. It is apparent that the partition coefficientsof Ba, Sr, and the REEs betweenminerals and melt significantly increasewhen the system becomessaturated with chlorine-rich aqueousfluids. The results of this study have implications for the petrogenesisof pegmatitesand other aplites becausemany have depletion and enrichment characteristicssimilar to the aplite and granophyre at Notch Peak. It is shown that rocks that are the result offractionation offanhydrous melts can be distinguished on the basis of their major- and trace-element chemistries from those that crystallized in the presenceof aqueous fluids or those that crystallized directly from the fluids. INrnonucrroN to modeling crystaVmelt processesmainly in mafic and The common associationof pegmatites,aplites, and intermediaterocks(Arth,1976; Hanson, 1980).Recently granophyreswith granitic rocks has been recognizedfor published experimental data on partitioning of several a long time, but it was the work of Richard Jahns and trace elementsbetween minerals and fluid and melt and coworkers that demonstratedthe importance of aqueous fluid (e.g., Carron and Lagache, 1980; Volfinger, 1976) fluids in the petrogenesisof these rock types. Jahns and also permit the modeling of crystallization processesthat Burnham (1957, 1969) suggestedon the basis of experi- involve fluids. mental work that pegmatitesand aplites crystallize when The purpose of this paper is to model the petrogenesis a separatesupercritical aqueous-fluid phase forms during of aplitesand granophyresassociated with the Notch Peak the late stagesof crystallization of a granite. The time of granitic stock, House Range,western Utah, and to dem- separationdepends on the solubility of fluids (e.g.,water) onstratethe feasibility of usingtrace elements in modeling in the magma at a given confining pressureon the system. the petrogenesisof pegmatitic rocks. Pegmatites in the A review of pegmatite petrogenesis(eerni, 1982) shows Notch Peak stock occur only as occasionalpods; thus, it that pegmatitesand associatedrocks have been described is difficult to obtain a representativewhole-rock sample. primarily in terms of mineralogy and major-element It is, however, easierto model the petrogenesisof aplites chemistry;however, the abundancesoftrace elementshave and granophyresbecause they can be sampled as whole not been generally used to their full potential to further rocks. Furthernore, the clearly defined mutual relation- elucidate the petrogenesisof theserocks. ships among the aplites, granophyres,and the host intru- The use of trace elementsto model the petrogenesisof sion can be usedto constrainpossible petrogenetic models. igneousrocks has been extensive but generally confined In addition, the petrogenesisofthe stock and the nature 0003{04x/86/03044460$02.00 460 NABELEK:TRACE-ELEMENT MODELING 461 A, granoPhYre NW Fig. 2. Cross-sectionthrough the granite body. Note the 1o- cation ofthe granophyrebelow limestone beds. El camb,isns€drmontary n- of approximately ,l-6 km. "**" l. I limestones and argillites at a depth UTAH The stock is a composite body consisting of a granite and two ditrerent intrusions ofquartz monzonite. The crosscuttingrela- Fig. L Geologic map of the Notch Peak stock with locations tions indicate rhat the granite intruded first, followed by quartz granophyre of the aplite and samplesdiscussed in this article. monzonite I and then by quartz monzonite II (Fig. l). AIl three lithologic units are texturally and mineralogically similar. The granite tends to be porphyritic (Fig. 3a), while quartz of fluid flow in the Notch Peak complex are well under- monzonite I has a seriatetexture and is mineralogicallyand tex- stood from previous studies (Nabelek et al., 1983, 1984, turally the most homogeneousof the three intrusions. Quartz II is porphyritic near the contact with quartz mon- ms.). monzonite zonite I but becomesseriate toward the core.Euhedral oligoclase, AN.cl,YTrcAl, METHoDS equant qrurtz, and minor biotite crystallized early in the para- genetic sequencein all three lithologic units. Although perthite The analysesof Si, Ti, Fe, Mn, Ca, K, Nb, Cu, Ga, Y, and Pb grains are the largest, they apparently crystallized late in the were conducted at Battelle-Northwest, Richland, Washington, parageneticsequence. Titanite, apatite, and zircon are accesso- using an energy-dispersiveX-ray fluorescenceKevex Model 8 l0 ries. The core ofquartz monzonite II containsextensively chlori- systemwith zirconium and silver secondarysources. Al, Mg, and tized biotite and sericitizedplagioclase. Nabelek et al. ( I 983) have Na were analyzed on samplesdissolved in lithium metaborate shown that this alteration is primarily due to interaction with flux by atomic absorption.U.S. GeologicalSurvey standards were fluids that exsolved during crystallization from deeper parts of usedas working standards.Replicate standardand sample anal- the magma chamber. The presenceof a free fluid phase is also ysesyield uncertainties of less than 100/ofor Mn, Nb, Cu, and suggestedby miarolitic cavities, containing quartz and micro- Ga; 590for Si and Ti; 30lofor K, Ca, Al, Na, Mg, Pb, and Y; and cline, that are especiallycommon in the granite. In addition, the lolofor Fe. granite is cut by quartz veins, one about 3 m across.The petro- Other elementswere analyzed by instrumental neutron acti- geneticmodeling of major- and trace-elementdata from the stock vation analyses(INAA) of 0.5-1.0-g irradiated aliquots of about (Nabelek et al., ms.) suggeststhat the three intrusions are coge- 5-kg homogenizedwhole-rock powders. The gamma-raysemit- netic and representvarious stagesof crystallizationand emplace- ted from the sampleswere counted by Ge(Li) detectorsusing the ment of a chemically fractionated diapir. sequentialcounting technique describedby Laul (1979). Zn, Sb, The granophyresare located at horizontal contactswith sedi- Rb, Sr, and Zr data wereobtained by coincidence-noncoincidence mentary rocks. Particularly good examplesare the samples166- counting, which resulted in reduction of the Compton back- 1A and 166-18. These sample were collected from an approxi- ground. Corrections were made for Ia, Ce, Nd, Sm, and Zr formed mately l-m-wide granophyre that occurs along the horizontal by fission of235U and as activation products of23EIJas a result contact betweenthe granite and the limestone layers of the wall of irradiation. U.S. GeologicalSurvey standardsGSP-I, BHVO- rock which form the roof of the magma chamber (Fig. 2). The I , and International Atomic EnergyAgency standardSoil-5 were location of the granophyrenear the top of the granite intrusion used to monitor the accuracyand precision of the results. The is similar to that in other plutons, including gabbros (e.g.,Car- one-standard-deviationuncertainties on replicate standardanal- michael et al., 1974).The granophyreconsists solely ofgraphic ysesare l-2o/o for Na, Ce, Co, Sc, and Zn; 3o/ofor Ba, Eu, Tb, intergrowths of alkali feldspar and quartz (Fig. 3b). The contact and Rb; 4-5o/ofor Sr, La, Sm, Yb, Lu, Hf, Ta, Th, U, and Cs; between the granophyre and sedimentary layers is sharp. The and less than l0Yo for Nd and Zr. K, Na, Rb, Ba, Sr, and Zn contact between the granophyre and the granite is about l0 cm wereanalyzed by more than one method. The replicatesare well thick and is characterizedby rhythmic, about l-cm-thick grano- within the stated analytical uncertainties. For consistency, only phyric and aplitic layers (Fig. 3c). The fine-grained aplitic layers the INAA data are reported. (not to be confused with the aplite dikes discussedbelow) appear Gnor,ocy to have acted as nucleation sites for fanning-out alkali feldspars with intergrown quartz. The $owth direction of the feldspars is The geologyofthe Notch
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