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Petrogenesis of the Concord Gabbusyenite Complex, North

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Petrogenesis of the Concord Gabbusyenite Complex, North Ameican Mineralogist, Volume 68, pages 315-333,1983 Petrogenesisof the Concord gabbusyenite complex, North Carolina BnnsnRA A. Orsr,Nr. Hanny Y. McSwBeN. Jn. Department of Geological Sciences University of Tennessee Knoxville. Tennessee37996 eNo Tnoues W. SeNoo Department of Earth and Planetary Sciences Massachusetts Institute of Technology Cambridge, Massachusetts 02139 Abstract The Concord intrusive complex, Cabamrs County, North Carolina, consists of a large gabbro stock envelopedby a horseshoe-shapedsyenite ring dike. The complex is clearly delineated from surrounding rocks by magnetic and radioactivity anomalies. Gravity modeling suggeststhat the gabbro pluton extends to a depth of about 6 km and that the syenitering dike dips steeply outward at an angleof 78" on the west side and dips vertically on the east side. A Sm-Nd mineral isochron for gabbro and a Rb-Sr isochron for syenite indicate that the two lithologies were emplacedcontemporaneously -405 m.y. ago. Initial Nd and Sr isotopic compositionsof gabbro and syenite in this complex overlap, suggesting the two rock types may be related through closed-systemfractional crystallization. Three gabbroic rock types are distinguished,based on the presenceofdifferent cumulus minerals: olivine-plagioclase, olivine-clinopyroxene-plagioclase,and clinopyroxene-pla- gioclase. Some magnetite and ilmenite occur as inclusions in these early phasesand are probably cumulus as well. A11gabbros contain postcumulusorthopyroxene and hornblende (tclinopyroxene), as well as minor biotite, sulfides, and apatite. The syenite is a seriate porphyry with megacrysts of perthitic feldspar in a groundmass of albite, microcline, clinopyroxene, amphibole, biotite, quartz, Fe-Ti oxides, pyrite, apatite, zircon and monazite. Mafic mineral compositions in gabbro and syenite indicate that fractionation involved only limited Fe-enrichment. Geochemicalmodeling of major and trace element variations indicates that the Concord syenite could have been produced by fractionation of olivine, plagioclase,clinopyroxene, and minor Fe-Ti oxide minerals, of the samecompositions as observedcumulus phasesin the Concord gabbro, from a basaltic parental magmasimilar in compositionto the Concord gabbro. This gabbro-syenite trend probably represents differentiation of a tholeiitic basaltic melt near a thermal divide along the critical plane of silica undersaturation.This fractionation took place in a magma reservoir at depth, before intrusion of the syenitic residual liquid into the fractured region between previously intruded gabbro and country rock. Introduction plex consists of a large gabbro pluton partially enclosed paper The Concord intrusive complex in Cabamrs County, by a ring dike of syenite. This will demonstratethat North Carolina, offers the best exposure of the gabbro- these intrusive bodies are related temporally as well as previously (e.9., syenite association in the southern Appalachian Pied- spatially. It has been inferred Butler and mont, among the several examplespreviously described Ragland, 1969)that the syenite is a later differentiate of parental produced gabbro; from this region(Medlin, 1968;Wilson, l98l). This com- the same magma that the this study substantiatesthat idea using petrologic observa- tions, chemical models, and isotopic constraints. ' Presentaddress: Gulf Oil Exploration and Production Com- The Concord intrusive complex has been a focus of pany, Bakersfield, California 93302. geologic interest for many years. The rocks of the Con- 0003-004v83/03M-0315$02.00 315 316 OLSEN ET AL.: CONCORD GABBRO-SYENITE COMPLEX cord area were first described by Watson and Laney Nockolds (1954)were used to correct the Fe values in the (1906)in a report on the ornamentaland building stonesof analyses. Cabamrs and Mecklenburg counties. Preliminary geolog- One fresh sampleof gabbro containing abundant clino- ic maps of the Concord complex were preparedby White pyroxene (CFG-5 in Table l) was selectedfor age-dating and Mundorff (1944),LeGrand and Mundorff (1952),and by the Sm-Nd mineral-isochron method. After crushing Bell (1960);the latter study also describedthe economic and sieving to obtain a 25-1fi) @ size fraction, crude potential of the area. Airborne magneticand radioactivity separationofplagioclase and pyroxene was effectedusing surveys ofthe Concord area were conducted by Johnson a Frantz magneticmineral separator. 130mg of pyroxene and Bates (1960),and Morgan and Mann (1964)reported a and 240 mg of plagioclasewere hand-picked from these detailedgravity study of the area. Bulk chemical analyses separatesfor analysis. Only clean, inclusion-free plagio- of Concord gabbro sampleswere given by Cabaup(1969), clase was accepted. Some opaques were unavoidably and average analyses of gabbro and of syenite were included in the clinopyroxene fraction due to the intimate reported by Butler and Ragland (1969). Price (1969) intergrowth of these two minerals, but material contain- analyzed trace elements in both gabbro and syenite ing inclusions of any other mineral was avoided. After samples.The crystallization age of the Concord syenite washing in 2.5 N HCI for one hour the separateswere was measuredby Fullagar (1971).These geophysicaland dissolved in an ultra-pure HF-HNOTHCIO+ mixture, geochemicalstudies will be reinterpreted in light of new and a split of the dissolved material was spiked for field, petrologic, and geochemicaldata presentedhere. isotope-dilution determination of Sm and Nd concentra- tions. lfi) mg each of powdered whole-rock gabbro CG-5 Analytical techniques and sources of data and syenite CS-0 were also dissolved for analysis. Sepa- Thin sections of Concord gabbro (10fi) points) and ration of Sm and Nd followed a two-column cation- syenite (500 points) were point-counted for modal analy- exchange technique described by Zindler et al. (1979). ses. In order to distinguish plagioclasefrom alkali feld- Analyses were performed on the f-inch automated mass spar, the syenite thin sections were stained with sodium spectrometerat the MassachusettsInstitute of Technolo- cobaltinitrate solution. Modal percentagesare reported in gy.Nd isotopiccomposition measurements were madeon Table 1. The minimum error of the modal determinations the unspiked fraction of samples.Procedural blanks for ranges from l-llVo relative using the chart of Van der this methodare less than 0.1 ng for both elements,and no Plas and Tobi (1965); actual analytical error may be blank corrections were required for any of the measure- greater due to relationships between adjacent grains and ments. Analytical precisionsas judged by replicate analy- to layering (Neilson and Brockman, 1977). ses of standards are 0.5Vofor SmA.{dratios and 0.003- tr3tr167r++19d. r43Nd/r4Nd Mineral compositions were determined using an auto- 0.004% 1or values of reported mated MAC 4005 electron microprobe at the University here are relativeto 0.512630for U.S.G.S. standardrock of Tennessee, utilizing the correction procedures of BCR-1. Bence and Albee (1968) and the data of Albee and Ray Field relationships (1970). Analyses using a slightly defocussed electron beam (-5-15 g.m diameter) were performed on minerals The Concord intrusive complex occurs within the in 9 polished thin sections.Operating conditions were set Charlotte belt of the southeasternPiedmont. The rocks of at 15 kV and 0.03-0.05g,a. beam current, dependingon the Charlotte belt have experienced Paleozoic regional the mineral to be analyzed. For amphibole analysesFe3* metamorphismof greenschistto amphibolite facies grade was calculatedusing the method of Leake (1978),and for and are intruded by a bimodal suite of gabbroic and oxide analysesFe3* was calculated based on stoichiom- granitic plutons. According to Bell (1960), the Concord etry. Representativeanalyses of major minerals are pre- pluton is surrounded by "massive and foliated dioritic sentedin Table 2; a complete compilation of microprobe and mafic rocks", with "granitic and granodioritic analysescan be found in Olsen (1982). gneisses"in contact with the syenite body on the east and Bulk chemical analysesfor major elements in 40 Con- west. Outcrops of country rocks near the complex are cord gabbro sampleswere obtained from an unpublished extremely scarce, but observations of saprolite indicate thesis by Cabaup (1969). Butler and Ragland (personal Bell's map units to be generally correct. communication, 1980)supplied individual bulk chemical The Concord complex consists of a discontinuous analysesfor major elementsfor 7 Concord syenite sam- syenite ring dike with varying thickness that partially ples and 6 Concord gabbro samples(previously published encircles a gabbro body of roughly oval shape. Both in summary form by Butler and Ragland, 1969), and lithologies are cut by numerous small pegmatite and less chemical analysesfor trace elments on the samesamples abundant diabase dikes. Field mapping of the Concord were taken from an unpublished dissertation by Price area was carried out using portions of the Concord, (1969). All bulk analyses reported total Fe as Fe2*; for Southeast Concord, Kannapolis, and Harrisburg Quad- normative calculationsFezOl/(FeO+FezO:) ratios for av- rangleU.S.G.S. I :24,fi)0scale topographic maps. Figure eragegabbro (0.24)and averagesyenite (0.45)taken from 1 is a geologicmap basedon this work and modified from OLSEN ET AL.: CONCORD GABBRO_SYENITE COMPLEX 3r7 Table l. Modal variations (volume %) ConcordGabbro PIg Hbl Bio Opq Cpx Opx Olv Apa Serp chl Ser 7ir .|6.1 LU-4 04. | 5.5 0.4 1.4 10.9 - 0.7 0.4 0.3
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