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-plagioclase. 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 observations, 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 concentrations. 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 0.2 cG-5 55.8 2.5 4.6 6.9 15.8 12.1 - 0.4 0..l 0.7 u. I cG-6 69.5 7.1 0.6 2.7 5.5 7.9 5.0 0.2 0.4 0.7 0.3 0.1 .l.9 cG-9 52.1 24.7 0.9 5.9 0.7 |0.3 2.7 0.6 0.2 CG-l0a 51.3 ?9.6 2.2 5.1 7.2 3.0 0.9 0-7 't.l CG-]0b 46.9 26;3 18.5 s.8 - 0.4 0.4 0.2 0.4 cG-lt 61.6 5.0 4.3 L2.8 14.0 1.0 0.3 0.3 0.6 0.1 cG-22 45.2 36.6 5.5 0.6 4.5 0.1 0.7 0.1 2.8 2.9 cG-23 52.1 30.3 0..| 2.5 3.0 8.8 0.9 0.4 1.8 - 0.1 cG-25 57.3 24.1 0.2 3.0 1.8 9.6 3.0 0.3 0.1 0.3 0.3 cG-26 47.2 33.3 0.1 3.3 2.7 10.8 1.5 0.5 0.6 '1.9 '12.4 cG-27 58.9 lt.7 0.r 13.8 0.3 0.2 0.2 0.5 '| cG-33 59.3 20.2 2.4 ?.2 9.1 1.7 - 0.2 .6 0.? cG-47 46.2 36.8 0.4 3.2 0.7 ll.8 - 0.2 0.4 0.3 cG-50 68.9 4.0 2.3 3.0 3.6 i6.l 0..l 0.5 0.5 1.0 cG-53 62.8 5.6 1.5 ?.2 2.9 10.5 13.4 0.2 0.4 0.1 0.4 cG-54 47.5 16.7 l.b 4.5 2.6 11.4 14.6 0.2 0.4 0.5 n? r,u-c/ o5.J d.u 4.5 3.5 0.8 12.8 6.2 0.3 0.5 .l.3 cG-58 65.8 3.7 3.2 12.9 7.4 4.9 - 0.1 0.5 0.2 cG-73 62.4 1.9 1a 4.0 14.2 7.2 7.4 - 0.1 0.2 0.9

cG-74 64.9 7.3 J, I 5.6 10.2 6.9 0.9 0.2 0.4 0.3 0.2 cG-81 58.5 7.9 4.0 5.2 12.2 11.8 - 0.3 0.1 cG-84 65.5 3.4 5.5 6.0 15.3 3.5 0.3 0.1 0.3 - 0.1 cG-87 64.5 14.7 4.4 1.0 10.8 2.7 0.1 0.2 0.8 0.6 0.2 cG-90 64.s 10.4 1.6 2.8 0.2 13.7 5.1 0.2 1.2 0.3

cG-92 62.2 15.8 l.) 2.7 6.6 7.7 2.9 0.4 0.2 cG-98 58.4 4.2 8.2 10.8 11.2 6.4 - 0.4 0.4 cG-102 50.6 17.6 3.5 12.2 10.7 4.7 - 0.1 0.3 0.3 cG-104 53.0 25.8 2.0 2.4 0.4 15.0 0.4 0.3 0.5 0.2 cG-105 5i .0 21.3 2.3 8.9 15.7 - 0.3 0.4 0.1 CG-106a53.9 16.2 3.9 14.9 ]0.5 - 0.4 u. I 0.1 cG-106b54.3 24.4 5.2 4.7 10.5 - 0.4 0.5 cG-]07 51.8 23.7 6.5 17.7 8.9 - 0.2 0.1 0.6 0.5

ConcordSyenite Plg Hbl Bio opq Cpx Ksp Prth Qtz Apa 71r

cs-39 4.3 1.3 0.2 2.6 2.6 3.4 82.4 2.6 n6

cs-62 4.8 3.1 0.2 a.t 4.2 3.1 77.0 4.0 0.4 0.4 cs-88 3.2 3.2 57.0 36.1 0.4 'I cs-2 0.5 0.4 0.4 to E1 0.5 86.6 0.7 .t 0.9 0.9 ,|.8 cs-37 18.6 - 1.8 2.7 1.8 68.5 3.9 0.4 0.6 .|.0 'f CS-20a 4.8 0.6 3.0 4.2 .5 79.8 3.2 1.0 0.8 0.2 3lE OLSEN ET AL.: CONCORD GABBRO_SYENITE COMPLEX

Bell (1960),White and Mundorff(1944), and LeGrand and mapped outcrop pattern of the complex. The first is an Mundorff (1952).The latter maps,which showthe syenite aeromagneticand aeroradioactivity survey of the Con- body to be continuous at the surface across the northern cord quadrangleby Johnson and Bates (1960). On their end of the pluton, are in disagreementwith the map of magnetic map, a group of closely-spacedcontours coin- Bell (1960),which denotesa parenthesesshape for the cides with the inner boundariesof the syenite body. The syenite body, open at the northern and southern ends. cause of the 45fi) gamma anomaly that surrounds the Syenite outcrops and quarries occur at the nothern end, gabbro is not clear, though Johnson and Bates suggested confirming the horseshoe shape described earlier by that magnetite might be concentrated along the contacts White and Mundorff (1944). between the gabbro and syenite. Three smaller gabbro The Concord gabbro is exposed as large, spheroidally bodies extendingout from the southern end ofthe pluton, weathered,black boulders. The syenite occurs mainly as the northernmost of which is shown in Figure 1, also pedestal-shapedboulders, and quarries provide abundant coincide with magnetic anomaly patterns. Johnson and fresh samples.The syenite outcrops are much larger than Bates' radioactivity map shows the syenite body outlined the gabbro boulders and are more highly weathered.Due by two parenthesis-shapedareas of high radioactivity on to the paucity of outcrops, lithologic contacts have not the easternand western margins.This feature undoubted- been observed,but can be fixed rather closely because ly influenced Bell's (1960) interpretation of the syenite the syenite forms a distinct topographically high area outcrop pattern, and may suggestthat the syenite to the (-100 ft. in reliefl relative to gabbro and country rocks. north of the Concord gabbro is much thinner. These two units can also be recognized from saprolite: The second geophysical report is a detailed gravity the gabbro forms dark gray soils of the Mecklenburg and study of the Concord Quadrangleby Morgan and Mann Iredell soil series, whereas light-colored arkosic soil is (1964).This work included calculationsof Bouguer anom- produced from weathering of syenite. There is additional alies, which were displayed on maps showing the simple geophysical evidence which is useful in determining the Bouguer anomaly and the residual anomaly; appropriate location of contacts at the surface, as well as suggesting portions of both maps are shown in Figure 2. On the the geometry of the pluton at depth. simple Bouguer anomaly map (Fig. 2a),the gabbro intrusion shows a pronouncedincrease in Bouguer value, Geophysical expression reachinga maximum of +35.5 milligals in the center. The The results oftwo geophysicalstudies conducted in the simple Bouguer anomaly map reflects large-scaleregional Concord area are available for comparison with the trends and deep-seatedmass distributions and can be

Table 2. Representativemicroprobe analysesof minerals

Gabbro Syeni te Gabbro Gabbro Cl i nopyroxene Cl i nopyroxene 0rthopyroxene 0livi cG-10t51 cG-s0t6l cs-12t51 CS-51tsl cG-33t71 CG-9t81 cG-9t8l cG-73t41

si02 51.3(6) 51.3(12) 53.1(4) s3.7(4) 53.8(4) 54.4(4) 37.4(2) 37.8(4) ',I.03(32) Ti02 0.59(5) 0.59(23) o.zL(z\ 0.08(2) 0.r2(?) A1203 2.2e(20) 3.76(60) 1.36(il) 0.64(6) 1.38( 7) 1.4e( e) FeO 8.51(26) 6.71(62) 8.70(35)10.4(2) 16.4(1) 15.6(2) 26.5(2) 23.6(1) MnO 0.37(4) 0.14(l) r.32(2) 1.87(l3) 0.48(2) 0.41(3) o.5o(4) 0.57(3) Pls0 15.0(2) 15.6(4) ls.1(3) 12.2(2) 26.5(3) 27.6(2) 35.4(3) 38.1(2) caO 21.1(2) 21.3(7) ls.4(2) 1e.8(1) 1.00(e) 0.82(r3) Na20 0.50(2) 0.52(11) 0.69(3) 1.43(6) 0.03(1) 0.03(2) Total 99.72 100.36 t00.26 100.20 99.72 100.38 99.80 100.07 Cation Basis(0) 6 6 66 66 44 'I si 1.917 1.888 1.969 2.013 .958 1.953 0.996 6.989 Ti 0.016 0.028 0.016 0.005 0.00? 0.003 Al 0.100 0.163 0.059 0.027 0.059 0.062 Fe 0.265 0.206 0.269 0.326 0.500 0.467 0.591 0.516 Mn 0.011 0.003 0.041 0.059 0.014 0.011 0.011 0.012 Mg 0.833 0.854 0.832 0.681 t.437 1.475 1.404 1.489 0.845 0.839 0.770 0.794 0.038 0.030 Na 0.035 0.037 0.049 0.103 0.002 0.002 OLSEN ET AL.: CONCORD GABBRO-SYENITE COMPLEX 319

Table 2 (continued)

Gabbro Gabbro Gabbro Syenite Amphibole Amphibole Bioti te Bioti te cc-sot3l cG-73t41 CS-43[6] CS-51t51 cG-5t41 CG-73t41 cs-51t51 cs-12[5]

si 02 41.6(2) 4l. s( 2) 4e.6(6) 50.8(4) 37.4(22) 37.6(6) 40.4(8) 38.e(12) '1.28(17) Ti02 3.l s(10) 4.04(11 ) 0.e2(6) 6.84(22) 3.76(38) 2.10(23) 5.1r(14) '|r.e(2) At 1r.e(t) 3.e3( 7) 3.09 ( 2) 14'1(2) 16.4(e) 11.5(4) 12.8(4) "0^ *Fero, 4.70 | .70 10.3 9.34 Fe0 4.95(2d e.63(2e) 3.58(2e) 3.1s(il) li .4(2) 7.e0( 60) 12.e(6) 14.3(2) Mn0 0.18(3) 0.2r(2) 1.s0(10) 1.76(7) o.05 (4) o.oo 0.61(s) 0.35(4 ) Mso ls.6(3) 13.3(1) 16.1(3) 16.7(2\ ls.s(3) le.2(lo) 18.1(4) 16.0(3) Ca0 1r.8(2) 11.5(1) 10.3(4) e.84(r4) NaZ0 2.64(1o) 2.41(11) 1.e3(16) 2.41(r0) o.2o(4) o.ss(16) o.oe(1) 0.14(3) Kzo 0.84(1) 1.13(5) 0.56(4) 0.67(21 9.5s(7_) 8.73(74\ 9.12(24) e.0s(39 Total 97.29 97.24 99.12 98.75 95.08 94.23 94.76 vb.5u 'I CationBasis 23 ZJ .J IJ 1l Il I Il (0,0H, F )

5l 6.072 5.139 7.076 7.297 z,toz 2.735 2.997 2.850 Il 0.346 0.450 0.137 0.068 0.376 0.205 0.115 0. 280 A1 2.048 2.073 0.660 0.475 r.229 t.407 I .000 1.108 fe 0.516 0.189 0.109 I .067 t+ fe 0.604 L.\tz 0.427 0.381 0.699 0.479 0.798 0.874 Mn 0,022 0.026 0.181 0.261. 0.002 0,000 0.037 0.021 'I i\49 J. JYJ 2.932 3.411 3.451 .706 2.077 1.999 1.743 I .840 1.861 7.576 1.507 Na 0.747 0.692 0.534 0.596 0.028 0.081 0.013 0,019 K 0.156 0.2I3 0.102 0.109 0.898 0.808 0.861 0.846

Gabbro Syenite Syenite Gabbro Gabbro Gabbro Plagi oci ase Plagi oc1 ase Perthite PI eonaste Magnetite Ilmenite

cG-50t71 CG-stel cs-12t51 CS-43t71 LU-C TJJ cG-5t3 l LU-9t3t

si02 s1.4(r2) se.1( 2o) 6s.1(6) 66.3(10) Ti02 0.13(3r) 1.06(8) 5i.e(il) A1203 3i,i(6) 25.2(s) 21.4(2) Al ^0^ oz,otI t., 1e.8(2) a3 o.6s(5) o.oe(1) FeO 0.20(3) 0.21(4) 0.2s(27) 0.14(s) Cr^0" o.le ( 10) 0.28(2) 0.02(2) ZJ cao 12.8(8) 7.63(t3e) 2.77(12\ 0.17(12) *Fer0, 4.68 58.6 4.82 Na20 4.02(55) o.or(yJ, e.63(3e) 4.se(352) Fe0 18.8( 6) 3e.e(lo) 37.s(2) K.0 0.13(2) 0.57(16) 0.27(26) 10.0(48) Mn0 n ,1/(\ 0.14(2) 3.13(ro) a- 14.6(5) 2.91(ro) Total 99.67 99.29 99.46 100.48 Mso 0.37(2) Total I 00.52 lot.06 100.39

Cati on Cation Basis (0) 8 8 8 8 Basis (0) 4 J

Si 2.341 a,oaI 2.877 2.996 lt 0.003 0.031 0.960 Al t.oo/ 1.117 1.000 AI 7,922 0.028 0.002 Fe 0.006 0,008 0.010 0.005 LT 0.004 0.008 0,622 0.367 0.131 0.008 fe- 0.092 t.709 0.089 Na 0.3s4 0.575 0.825 0.401 l- e- 0.395 1.294 0.771 K 0.006 0.031 0.014 0.576 Mn 0.005 0.005 0.065 Mg 0.569 0.021 0.107

All Fe is reportedas FeOexcept for amphiboles,in which Fero" was calcu'latedby the methodof Leake(1978), and for oxides, in which FeZ03was calculated from stochiohetry.

Numbersjn brackets [] are replicate analysis averaged. Numbersin parenthesesO indicate one standarddeviation in tenns of least units reported. t20 OLSEN ET AL.: CONCORD GABBRO-S YENITE COMPLEX

ro'43'30" 80'3a'00" 35"26'30" ffi Concord Gobbro ffi Concord Syenite Gneissic,Grqniticond Gronodioritic Rockr of ffi the Chorlotte Belt

(V) Dioritic qnd MoficRockr 14 oI rhe Chorlorte Belt

3s" 17'!0"

Fig. l. Geologicmap of the Concord intrusive complex, a compilation of this work and that of White and Mundorff(1944),LeGrand and Mundorff(1952), and Bell (1960).The inset map showsthe location of the complex within the Charlotte belt in the North Carolina Piedmont.

(o) (b)

o_2 4

)}o o s'u ffiffi \ Syrniio Gobbro

ContourInlcrvol { Milligolr

Fig. 2. Contoured gravity data (Morgan and Mann, 1964)superimposed on the surface outcrop pattern of the Concord gabbro- syenite complex. (a) Simple Bouger anomaly map. (b) Residual anomaly map. Line A-A' in (a) was used to produce the gravity profile in Fig. 3. OLSEN ET AL.: CONCORD GABBRO-SYENITE COMPLEX 321 used to estimate the shape of the body at depth. The residualanomaly map (Fig. 2b) representsthe gravitation- CGINCCIFIEI GAEIBFIG' al distribution of local, shallow geologic structures. The Stm-Nd min€rEl isochron contours shown on this map roughly outline the shapeof pluton O the at the surface. The trend of isolated gabbro z bodies extending from the open southern end of the complex is reflected in both gravity maps, suggesting o z,^ these satellite intrusions are related to the main gabbro oii body at depth. Pl.9 The detailed nature of this gravity study provides an aGE:4O7i36my opportunity to construct a gravity profile and propose a (143Nd/144Ndto: .ElegaE tas simple model of the geometry of the pluton at depth. An observedgravity profile was constructedalong line A-A', shown on the simple Bouguer anomaly map in Figure 2a. o-bg o04 g. oB @t2 6. 16 @2q This particular line was chosen becauseit produced the 147SM/ 1.44ND most symmetrical profile (Fig. 3). Using the Talwani 2-D Fig. 4. Sm-Nd mineral isochron for Concord gabbro, defined Gravity Program No. W9206 (Talwani et al., 1959), by plagioclase and clinopyroxene sepaxatesand whole rock several theoretical gravity profiles were constructed by (wR). varying the dip angleand direction of the syenite dike and the depth to the bottom of the pluton. Figure 3 illustrates the profile that most closely resemblesthe observed if the syenite were modeled as a wedge thickening down- gravity profile along A-A'. The model which produced ward rather than as a plate of uniform thickness. this profile depicts a steep-walledsyenite ring dike with an outward dip of 78' on the west side and vertical dip on Age dating and isotopic constraints the eastside, and a depth ofabout 6 km to the bottom of The three samples(clinopyroxene and plagioclasesep- the complex. An even more refined fit might be obtained arates,whole rock) usedto date the gabbro by the Sm-Nd technique give a near-perfect isochron age of 407-+36 m.y. (1o error, York, 1966), as shown in Figure 4. -'- Colculoted Sneeringer(1981) has measureddiffusion rates of Sm in o clinopyroxene which show that difusive exchange of o 3 rare-earthelements involving this mineral (the major host : of REE in the gabbro) is unlikely below magmatic tem- o peratures.Closure temperaturesabove 850'C are indicat- E o therefore C ed even for improbably slow cooling rates. We interpret the mineral isochron age as the emplacement o .: age of the gabbro. Rb-Sr whole-rock dating of the sy- ! enite, which also gives an emplacementage, was reported o c by Fullagar (1971) at 404-r2l m.y. (recalculated using ).Rb: 1.42x l0-rl/yr) and is identicalwithin error to our result for the gabbro. Measurementsare given in Table 3. E An important finding is that the Nd initial ratios as :1 taken from the gabbro isochron (Fig. a) and calculatedfor 3 the syenite using the published Rb-Sr age are indistin- c t guishable.The sameis true for Sr initial ratios (Fullagar, 1971).These observations are consistentwith a closed- system relationship between the gabbro and syenite and are strong evidence that the syenite was not derived by rl A ------I assimilation of any significant amount of crustal rock by Horizonlol Dirtoncr (3 Km Inrorvolr) magma similar in isotopic composition so that which formed the Concord gabbro. Such a processwould result in elevation of initial 87srf6sr of syenite relative to ffi gabbro if typical older intermediateto upper crustal rocks Diorilic And Gobbro were involved. Isotopic compositionof potential lower Mofic Rockr crustal contaminating material may be inferred from Fig. 3. Comparisonbetween observed and calculated gravity Hercynian granitic rocks of the Charlotte belt, which profiles along section A-A' in Figure 2a. The calculatedprofile probably derived a significant portion, if not all, of their assumesthe geometry and rock densities indicated. massfrom the lower crust. PublishedtTSr/eSr values for 322 OLSEN ET AL.: CONCORD GABBRO-SYENITE COMPLEX

Table 3. Results of isotopic analyses r (la3Ha/laato) (la7sr/laha) t) Sample neasured {l43*a/l44Nd)rnrar", "*o(

Concordgabbro CG-5 .512328+ 25 +4.4 + 0.5

plagioclase .512453+ 17 .0504

wholerock .512650+ 18 .1?12

cl i nopyroxene .512749+ 17 ,1574

Concordsyenlte CS-o .512595+ 20 .1049 .5l2318+ 372 +4.2 + O.g2

l.r' eNd\e'e,,(t) =- tilTilF4?il1.(143n4/l44Ha) sampre,t - ll x l04 Parametersused for calculating CHURevolution: CHUR,t = =-.512622 l47rrrl44ro .1967 (l43na/l44na)today 2. Error lncorporatesl-o errOr in Rb/Srage. All othererrors are 2-o.

these plutons (Fullagar, 1971;Jones and Walker, 1973; fractionation. The syenitic magma was not produced at Fullagar and Butler, 1979)indicate that Sr isotopeswould the present erosional level, because no intermediate be a poor indicator of crustal contamination at depth, rocks occur, but could have formed by fractionation of since several of them have low initial ratios, some very the source magma body at depth. A fractured region close to the value for the Concord gabbro/syenite.Inves- around the contact between the gabbro and country rock tigation of Nd isotopic composition of the granitic plutons was subsequently intruded by this syenitic magma. As has shown them to have strong "crustal" signatures,with noted by Morgan and Mann (1964),the syenite ring dike average initial eN6 = 2.5 (Sando and Hart, 1982).This probably did not form as a result of subsidence of a requires a lower crust at least as depleted as these values central gabbroic block into syenitic magma, becausethe with respect 1o tar5167t+a51dand is consistent with calcu- magnitudeof the residual gravity anomaly is inconsistent Iations of lower crustal isotopic composition based on with this geometry and because subsidence of many probable ageof the crust and reasonableguesses about its kilometers of the steep-walled gabbro stock would be Sm/Nd. These values are quite diferent from those required to provide room for a ring dike of the observed measured in the Concord rocks; assimilation of lower width. The incompletenessof the ring dike may be related crustal material would thus lower the l43Nd/l4Nd of an to the presenceof small, satellitegabbro bodiesextending assimilatingmagma with isotopic composition like that of from the open southernend ofthe pluton. The geophysi- the gabbro. Nd concentrations measured in the gabbro cal maps suggestthat these bodies may be continuous at (40 ppm) are probably lower than those of the potential depth with the Concord gabbro body, possibly represent- crustal contaminant, Even for the case where they are ing outcrops of a dikelike projection of gabbro. Subsol- equal, calculations using the granitoid isotopic composi- vus crystallization of alkali feldspars in the syenite tion for the contaminant result in a upper limit of 20% indicatesthe complex was emplacedat depths of approxi- assimilatedmaterial when errors in our initial ratio deter- mately 15 km or greater. We will now examine the minations are taken into account. The limit is conserva- petrographic and geochemical characteristics of gabbro tive, since the isotopic contrast betweencrust and gabbro and syenite in order to devise and test a fractionation may be greater, and becauseof the assumptionsof equal model relating the two lithologies, as proposed above. concentrations.The combined use of the Nd and Sr isotope systems therefore permits both upper and lower Petrology of the gabbro crustal contamination of the Concord parental magma to Concord gabbro samples are medium-grained meso- any large extent to be ruled out. and orthocumulates consisting primarily of plagioclase, olivine, clinopyroxene, orthopyroxene, and amphibole, A model for emplacement of the complex with lesseramounts of biotite, oxide and sulfide minerals, The identical crystallization agesfor the Concord gab- apatite, and alteration products. Three major gabbroic bro and syenite indicate that these units were associated rock types are distinguishedby the presenceof different in time as well as space.A reasonablesequence ofevents cumulus minerals. (Cumulus is used here only as a that would produce the structure of the Concord complex textural term to describe discrete crystals and does not begins with the intrusion of mafic magma to form a necessarilyimply settled crystals (Irvine, 1982)).The first gabbro stock. This pulse of magmawould be derived from type containscumulus olivine and plagioclasepoikilitical- a large reservoir of magma at depth prior to significant ly enclosedby postcumulus orthopyroxene, clinopyrox- OLSEN ET AL.: CONCORD GABBRO_SYENITE COMPLEX 323 ene, and hornblende (Fig. 5a); this type is referred to as compared using the classification diagrams devised by olivine-plagioclase cumulate (OPC) gabbro. The second Streckeisen (1973). The gabbroic rocks are norites and gabbroic rock type is a clinopyroxene-plagioclasecumu- gabbronorites, according to modal clinopyroxene/ortho- late (CPC) in which these phases are poikilitically en- pyroxene ratios (Fig. 6). CPC and OCPC samplesgeneral- closed by postcumulus orthopyroxene and hornblende ly plot within the gabbronorite field of Figure 6, whereas (Fig. 5b). In this rock type clinopyroxenedoes not occur OPC samples plot as norites and gabbronorites. For as oikocrysts. Samples which contain all three cumulus simplicity only the term gabbro with appropriate mod- phasesare olivine-clinopyroxene-plagioclase cumulates ifiers to indicate cumulus phases will be used here for (OCPC) and are rather uncommon. Areal distributions of classification. because the identification of cumulus OPC, CPC, and OCPC sampleswithin the pluton appear phasesis central to reconstructingfractionation patterns. to be non-systematic(Olsen, 1982). Plagioclaseis the dominant mineral in gabbro samples. Modal proportions of the major phases(Table 1) can be Individual grains are normally zoned and partially altered

2 mm. Fig. 5. Photomicrographsof samplesfrom the Concord complex in cross-polarized,transmitted light. (a) Cumulus olivine and plagioclasein OPC gabbro. (b) Cumulus clinopyroxene and plagioclaseand postcumulus amphibole in CPC gabbro. (c) Zoned perthite phenocryst with augite and Fe-Ti oxide inclusions in syenite. (d) Porphyritic texture in syenite, with perthite phenocrysts and groundmassofplagioclase, microcline, quartz, and interstitial augite, amphibole and oxides. 324 OLSEN ET AL.: CONCORD GABBRO_SYENITE COMPLEX

in composition but contain exsolution lamellae of clinopyroxene along (100). Orthopyroxene also forms thin reaction rims around some olivines and symplectic inter- growths with magnetite and ilmenite. This symplectite normally occurs at the edges of olivine grains that are surrounded by hornblende, an indication that the intergrowth is a result of reaction between olivine and residual melt (Ambler and Ashley, 1980;McSween, 1980).Plate- like lamellaeof ilmenite oriented along (010)also occur in the orthopyroxene. In OPC and OCPC'gabbros, orthopyroxene is more magnesianthan in CPC gabbros. Large oikocrysts of hornblendeare strongly pleochroic from reddish-brown to tan. Compositions of amphiboles i.t in terms of Ca, Mg and Fe are shown in Figure 8. 'rCPX According to the classification system of Leake (1978), the gabbroic amphiboles are pargasites and magnesio- hastingsites. These amphiboles appear to have formed o OPC Gobbro primarily by late-magmaticreplacement of postcumulus o CPC Gobbre pyroxenes. o OGPC Gobbre Biotite occurs as tabular sheaths or as rims around oxide minerals. Gabbroic biotites appearto show sympa- thetic enrichments of Ti and Fe (Fig. 8). Magnetite and ilmenite occur as interstitial subhedral o oo grains or as euhedral to subhedral inclusions in cumulus Olivinc and postcumulus minerals. Thus some oxide grains may {Gobbre)Noriro be cumulus phases. Composite magnetite and ilmenite grains are commonlyjoined by a boundary, along which a o thin discontinuousrim of pleonasteis found. Some mag- t netite grains contain thin straight lamellae of ilmenite. '.OLV{ olx ; Compositionsof typical grains are shown in Table 3. cPx Fig. 6. Modalvariations in gabbroicrocks. Nomenclature is that of Streickesen(1973).

to sericite. Plagioclase in the gabbro pluton ranges in composition from An77_31,but the range is much more restricted within any specific sample (Fig. 7). The most calcic plagioclaseoccurs in OPC gabbros, and the least calcic in CPC gabbros. Olivine within the gabbro ranges in composition from Foze-zo(Fig. 8), but grains have uniform composition within any one specimen.Olivine in OPC gabbrosis more magnesianthan that in OCPC gabbros. Most grains are subhedralto anhedraland are replacedalong fractures by magnetite and serpentine. Some grains are rimmed by orthopyroxene and hornblende. Clinopyroxene occurs as oikocrysts surrounding plagioclase and olivine in OPC gabbro, and as discrete Ab An subhedralto anhedral grains in CPC and OCPC gabbros. gabbro This mineral is diopsidic augite (Fig. 8), and grains within Fig. 7. Molar compositionsoffeldspars in Concord and syenite. Individual zoned plagioclasecore and rim compositions the same section all have the same composition. Clino- in gabbro samplesare joined by tie-lines alongthe Ab-An join. A pyroxene contain$ exsolution lamellae of orthopyroxene key for symbolsfor different gabbro types is given in Fig. 6. Bulk parallel to (100),as well as thin platelets of magnetiteand perthite compositionsin syenite(large open circles) arejoined to ilmenite parallel to (010). correspondinglamellae and host compositionsby tie lines along Orthopyroxeneoccurs as large oikocrysts in all gabbro- the Ab-Or join. Groundmassfeldspars in syeniteplot near Ab or ic rock types. These bronzites (Fig. 8) are homogeneous along the Ab-Or join. OLSEN ET AL.: CONCORD GABBRO-SYENITE COMPLEX 325

O.5Co host mineral and which is the exsolved phase. Some o OPC Gobbro perthitic feldsparsare strongly zoned, with perthitic cores by bands of sodic plagioclase,followed again r CPC Gobbro surrounded by perthitic rims. Many megacrystscontain dusty zones o OCPCi GobbroGobbr of sericite and clay inclusions. Clinopyroxene, apatite ,t Syonifof. and oxide minerals are present as inclusions. The bulk compositionsof perthite and antiperthite megacrysts(determined by averaging a number of defocussed beam analyses)are joined to their correspondinglamellae and host compositions by tie lines in Figure 7. The compositions of alkali feldspar and sodic plagioclaseare similar in perthites and antiperthites. Feldspars in the groundmass are plagioclase (Anr-ro) M9 \Nlii osFr and microcline. The compositionsof thesephases (Fig. 7) orv.\ tN.i i , are similar to those in perthitic megacrysts.Quartz occurs ri I within the groundmass, mostly as myrmekitic inter- ri ! tt I growths with albite. However, one analyzedsample (CS- 02 Ti ll 88. Table 1) contains sufficient anhedral quartz grains to be classifiedas granitic. /Jiti Mafic and opaqueminerals in the syenite tend to occur B.'/ as intergranular clusters between megacrysts. Clinopyr- * oxene grains are anhedral, fractured, and commonly rimmed by brown amphibole. A green amphibole occurs M9 Fe as discrete anhedral to subhedral grains. Syenite clino- Fig. 8. Molar compositionsof ferromagnesiansilicates in the pyroxenes are more Fe-rich than those in gabbros, and Concord gabbro and syenite. Minerals (clinopyroxene, syenite amphiboles have lower Ca contents than the hornblende,orthopyroxene, olivine, biotite) coexistingin the correspondingphases in gabbros (Fig. 8). These amphi- joined phases samesamples are by tieJines.All of these are boles are magnesio- and actinolitic hornblendes in the homogeneous. terminology of Leake (1978). Biotite is present in small amounts as ragged flakes clustered with other mafic minerals. Biotite in the syenite is pleochroic in shadesof gabbroic Sulfide minerals occur as round blebs in reddish-brown to brown or pale green to green; only the samples.Pyrrhotite, the dominant sulfide phase, is asso- reddish-brown variety occurs in gabbro' Biotite in syen- ciated with lesser amounts of pyrite, chalcopyrite and ites is more Fe-rich and Ti-poor than in gabbro (Fie. 8). pentlandite. Anhedral magnetite, ilmenite and pyrite grains are also Other accessoryminerals include apatite, which forms associatedwith mafic clots, along with accessoryapatite, euhedrallaths and prisms, mostly interstitial, but also as zircon. and monazite. inclusionsin feldspars.Minute zircon inclusions in biotite produce pleochroic haloes. Plagioclase grains contain history of the complex veinlets and patchesof sericite as an alteration product, Crystallization and chlorite and serpentine are present as alteration In OPC gabbro, cumulus olivine and plagioclase are products of pyroxene and olivine. enclosedby postcumulus orthopyroxene, clinopyroxene and hornblende. Clinopyroxene joined the other crystal- Petrology of the syenite lizing cumulus phases in OCPC gabbro. With further The Concord syenite is a coarse-grained,seriate por- fractionation olivine ceased to crystallize because of a phyry containing large phenocrysts of perthite and anti- reaction relationship with the melt to form orthopyrox- perthite (Fig. 5c) in a groundmassof perthite, plagioclase, ene. In CPC gabbro, clinopyroxene and plagioclaseare microcline, clinopyroxene, biotite, amphibole, quartz, cumulus minerals, surroundedby postcumulusorthopyr- oxide and sulfide minerals, zircon, apatite and monazite oxene and hornblende. This textural sequence implies (Fig. 5d). Modal proportions of the major minerals (Table formation of cumulate rocks in the following order: OPC, 1) fall within the syenitefield of the I.U.G.S.-approved OCPC, CPC. This sequenceis corroborated by Fe- classificationdiagram (Streckeisen, 1973). enrichment trends in ferromagnesianminerals and Na- Feldspar megacrysts range up to 2 mm in length and enrichment trends in plagioclasecores (Figs. 7,8). commonly stand out in relief on weathered surfaces of This evidence indicates that olivine and plagioclase syenite. Perthite and antiperthite can be distinguishedin were the first minerals to crystallize from the parental stained syenite thin sections and slabs, but in unstained magma.Inclusions of Fe-Ti oxides within cumulusphases samplesit is dfficult to determinewhich componentis the imply that these may have also crystallized early. The 326 OLSEN ET AL.: CONCORD GABBRO_SYENITE COMPLEX

crystallization sequencewas joined later by clinopyrox- Concord syenite was not derived by fractionation from ene. Postcumulusmaterial crystallized to form oikocrysts the Concord gabbro in situ, but both may have been of clinopyroxene and orthopyroxene in early cumulate derived from a common parental magma at depth. It is rocks, or orthopyroxene only in later cumulates. Horn- assumed that the magma that formed the gabbro was blende probably formed by late magmaticreplacement of extracted from such a chamber before significant frac- postcumuluspyroxene. Biotite also crystallized from the tionation occurred. Thus the identity and compositionsof magmalate in the sequence,or by reaction of oxides with the cumulus minerals in the Concord gabbro would be the residual melt. sameas those in the larger fractionating parental magma If the Concord gabbroformed from magmatapped from chamber,in the absenceofsignificant pressureeffects. (2) a larger reseryoir at depth before significantfractionation It is assumedthat the averagesof all analyzedsamples of had occurred, the gabbroic crystallization sequencere- gabbro and of syenite representthe bulk compositions of flects the subsequent crystallization sequence in the these two magmas. Because the gabbro samples are larger chamber as well. The fractionation of this parental cumulate rocks, this approximation is not rigorously magmathus may have been controlled by crystallization correct, especially for trace elements. of plagioclase,olivine, and clinopyroxene, possibly with In any chemicalfractionation diagramrelating Concord minor oxide phases.Because most samples(OPC and gabbro and syenite, there is a gap between clusters of CPC gabbros)contain only two of the three major cumu- points because there are no samples with intermediate lus phases it is difrcult to estimate the relative propor- compositions. For this reason the exact diferentiation tions of these three minerals in the fractionating assem- path between gabbro and syenite cannot be determined. blage. The averagemodes for each rock type and for all However, the identification of crystallizing phases that gabbro samplesare presented in Table 4; however, it is may have dominated the fractionation sequencecan be uncertain whether an average mode for all samples estimatedfrom major element chemistry and mineralogy weighs the various types of cumulates in their proper of the two end-members. Four representative two-ele- volumetric proportions. Olivine and clinopyroxene occur ment variation diagramsare illustrated in Figure 9 (a-d), approximately in a cotectic ratio (Table 4), but plagio- using averagegabbro and syenite compositions (Table 5). clase is overabundant.It is clear that some of the plagio- These variations have been interpreted using the extrac- clase is postcumulus, formed as zoned overgrowths on tion method of Cox et al. (1979).Average mineral compo- cumulus crystals or as crystals nucleatedfrom intercumu- sitions for olivine and clinopyroxene and the average lus liquid. The proportions of the fractionating phases plagioclasecomposition from the sampleswith the most may be better estimatedfrom the geochemicalarguments calcic plagioclase (Table 5) were used to construct the that follow. extract polygons (trianglesin Figure 9, a-d). The average syenite composition can be produced by fractionation of Fractionation of major elements plagioclase + olivine * clinopyroxene, as the extract The following necessaryassumptions underlie conclu- composition in each of these diagrams falls within the sions drawn from this and the following sections: (l) The polygon they define, but syenite cannot be produced by

Table 4. Relative proportions of cumulus minerals

OPc cPc ocPc Avg. I Cotectic , Extract i Mixing , Modes(20) Modes(7) Modes(6) Modes' Proportions' Diagrams" Progran'

Plag 86 81 84 84 5s-60 57-61 57 01v 7 - ll 5 10-15 13-25 19 *5 '10 Cpx 18 5 25-30 I 5-30 18

Mt 0.5 0.9 0.1 0.7 4

Iln 0.5 0.3 0.1 0.3 z

units in O representnumber of samplespoint-counted. values are recalculatedto 100%, '1. Averagemodes are weightedaverages of all gabbropoint counts. 2. Cotectic proportionsfrom McCaIIumg! al, ('|980). 3. Rangesof mineral proportionsestimated from FiEure l'le.

4. Computedmineral proportionsfrom the petrologic mixing calculation (Table 6). *AlI Cpx in oPCgabbro is post-cunulus. OLSEN ET AL,: CONCORD GABBRO-SYENITE COMPLEX 327 (o)

o o L'

(d)

cpt Fig. 9. (a-d) Four representativetwo-element variation diagramsshowing the inferred gabbresyenite fractionation trends (solid arrows). In each diagramthis trend can be produced by extracting (dashedlines) mixtures of plagioclase,clinopyroxene, and olivine with the same compositions as observed cumulate gabbro phases. Compositions of average gabbro, syenite, plagioclase, clinopyroxene, and olivine used in constructing these diagramsare given in Table 5. (e) Estimation of the relative proportions of plagioclase,clinopyroxene and olivine required to produce the gabbro-syenitetrend in eight extraction diagrams.The region in which the lines intersect (shadedarea) representsthe range of mineral proportions in the assemblagemost likely to have produced this fractionation pattern. 328 OLSEN ET AL.: CONCORD GABBRO-SYENITE COM?LEX

Table 5. Average compositions (weight Vo oxides)ofConcord gabbro and syenite and presumedcumulus phasesused in geochemicalcomputations

Syenite 0.'r9(0.02) 3.95(1.0s)4.41(1.23) 2.26(1.21',) r.0s(0.29) 7.45(0.49) 1.27(0.64) 17.r0(0.40)60.8(2.20) 98.48 Gabbro '17.59(l.80) 0.r4(0.03) e.73(1.88)O.3l(0.19) r0.47(1.02) r.20(0.38) 2.s',r(0.2e)9.59(2.00) 46.9(r.50) 98.M 0livine 0.49 23.95 38.07 37.54 100.05 Plagioclase2 - 0.24 0.23 11.76 4.43 29.45 53.19 99.30 Plas (An 70)3 - 0.23 0.08 14.04 3.29 3l.39 50.01 99.04 Plas (An 39)3 - 0.21 0.57 7.63 6.6l 25.20 59.07 99.29 Cli nopyroxene 't 0.46 .43 2t.23 0.85 0.55 l5.39 3.19 50.95 100.05 Itlagnetite - 90.51 3.54 0.50 0.77 - 95.32 Ilmenite - +t.oo 51.92 2.91 0.09 - 96.58

Averagesyenite and gabbro values frm data of cabaup(1969) and Butler and Ragland(1969). units in O representone standarddeviation. Averagemineral cmpositions from electron microprobeanalyses (0lsen, l9g2). l. Total Fe as Feo.

2. Averageof plagioclaseanalyses from samplewith mostca'lcic plagioclase (usedin F.igurell).

3. Mostsodic and calcic plagioclasesmeasured (used in the petrologic mixing prcgram,Table 6).

fractionationof any two of thesephases. This conclusion cumulateassemblage consisting of 57% plagioclase(aver- is in agreement with previous textural evidence that age An 62), 19%olivine, 18% clinopyroxene, 4Vomagne- suggestedall three minerals are cumulus phases in the tite and 2Voilmenite from the averagegabbro composition gabbro. The extraction diagramsare also consistent with to produce a residual melt ofthe averagesyenite compo- fractionation of a four-phase assemblageplagioclase + sition. The sum of the squaresof the residuals(0.053) is olivine t clinopyroxene + orthopyroxene, but this frac- low, and the error values are reasonable. The largest tionation is not consideredplausible becauseorthopyrox- error is in the Na2Ovalue (Table 6). To balanceNa2O, the ene in the Concord gabbro is postcumulus and probably fractionating plagioclase should be more calcic. This was never of major importance in the crystallizing assem- small imbalance is produced by the inability to balance blage. SiO2, possibly becauseof an analytical error in SiO2 or Each extraction diagram gives a range of proportions Na2O. The proportions of cumulus phasesgiven by this for eachmineral in the crystallizing assemblage.Figure 9e method compare well with the values determined from illustrates the proportional rangesgiven by 8 such varia- major element extraction diagrams and, considering the tion diagrams. This figure was constructed by determin- uncertaintiesin thesecalculations, differ little from cotec- ing the range of extract compositions given by each tic proportions (Table 4). extraction diagram, and plotting each range on an equilat- eral plagioclase-olivine-clinopyroxene triangle. The re- Fractionation of trace elements gion in which lines intersect (shaded)is taken to be the Figure l0 presentsaverage trace element contents and range of mineral proportions in the assemblagemost standard deviations for 6 gabbro and 7 syenite samples likely to produce the gabbro-syenite fractionation. (Price, 1969)versus Diferentiation Index, D.I. (norma- The validity of this fractionation model was put to a tive Q + Or + Ab + Ne + Lc). It is difficult to specify more quantitative test using the least squarespetrologic how fractionation afected these trace element contents mixing calculations described by Wr.ight and Doherty becausethe concentrations in the original liquid are not (1970).Average compositions of gabbro and syenite and precisely known. The average Concord gabbro trace the samecompositions of clinopyroxene and olivine used' element contents are taken to approximate those of the above (Table 5) were employed in the calculations. The parent magma.We attempt here only to determineif trace plagioclasecomposition was allowed to "float" between element patterns are qualitatively consistent with the the most calcic (An 70) and the most sodic (An 39) fractionation model discussedabove. observed values in the gabbro (Table 5). Addition of Fe- In Figure 10, Zr and the alkalis K, Rb and Li are Ti oxides to the calculation resulted in an improved fit; enriched in the syenite relative to the gabbro, whereasall textural evidence has already been cited that these may other analyzedtrace elementsare depleted.Fractionation have been cumulus phases as well. The computed solu- of a spinel phase and a sulfide phase are suggestedby tion (Table 6) calls for removal of 95% by weight of a depletion of Cr and V, and of Cu and Zn, respectively. OLSEN ET AL.: CONCORD GABBRO-SYENITE COMPLEX 329

Table 6. Solution to the petrologic mixing problem

0bserved Calculated Syenite 0livine Clinopyroxene l4agnetite llrenite Plagioclase Gabbro Gabbro Residuals si02 61.86 37.71 5l.l6 0.00 0.00 52.78 47.72 47.62 0.10

Alzog I 7.40 0.00 3.21 0.81 0.10 30.10 17.90 17.86 0.04 Feol 4.oz 24.06 7,47 94.96 43.14 0.23 9.90 9.86 0.05

Mgo 1.30 38.24 I 5.46 0.53 3.02 0.00 9.to 9.76 0.01 Cao 2.30 0.00 21.32 0.00 0.00 12.54 I0.66 10.67 -0.01

NaZo 7.58 0.00 u.30 0.00 0.00 A 1a ?.56 2,75 -0.19 u0 4.4e 0.00 0.00 0.00 0.00 0.21 u.5z 0.34 -0.02 Ti02 1.07 0.00 0.86 3.7? 53.76 0.00 1.23 Ll8 0.05

Total 100.02 I00.01 I 00.04 I 00.02 I 00.02 100.03 100.0s 100.04 [12=0.0533

Solution 5.01 18.1l 17.46 ?24 I .59 54.50(An 62)

Error2 I.l U.J 0.7 0.1 0.2 0.7

Compositionof l9 i8 4 z curnulateFractionr

1 Total Fe as Feo. z. Error estimatedby empiricalprocedure of Wrightand Doherty (1975).

J. Compositionrecalculated to I001.

That a spinel phase was part of the fractionating assemblage is suggestedfrom textural observations and petrologic mixing calculations. The bleb-shaped clusters of sulfide phases in gabbros suggest that an immiscible sulfide phase may have separatedfrom the liquid at an early stageas well. Two simple models describing the behavior of trace elements during fractional crystallization, Rayleigh (or perfect) fractional crystallization and equilibrium crystallization, have been used to constrain the degree of fractional crystallization and the identity and proportions of crystallizing phases required to produce a particular residual melt. The Rayleigh fractional crystallization equation produces a straight line on a logarithmic plot comparing two trace element concentrations, and the equilibrium crystallization equation produces a curved line. In Figure 1l(a,b), logarithmic plots of Rb versus Sr and Rb versus Zr in Concord gabbro and syenite are presented. In both casesthe gabbro-syenitetrend coincides with the calculated vector along which a melt fractionating plagioclase, olivine, clinopyroxene, and magnetite by Gobbro D.l, Syenifc Rayleigh fractional crystallization occurs. The least squaresfractionation calculation indicated that 95Vofrac- Fig. 10. Trace element variations between average Concord given cumulus assemblagewas gabbro and syenite versus Differentiation Index (D.I.) Data tionation by weight of the obtained by Price (1969)were averagedand tabulated by Olsen required to produce a melt of average syenitic composi- (19E2). Vertical and horizontal bars represent one standard tion from the average gabbro composition. The vector deviation. lengthsfor Rayleigh fractionation in Figure I I (a,b) are in 330 OLSEN ET AL.: CONCORD GABBRO-SYENITE COMPLEX qualitative agreement,indicating degreesof fractionation trace element concentrations in gabbro and syenite are of 85% and80Vo,respectively. An even better agreement qualitatively consistent with the proposed fractionation with calculations for major elements may be obtained if model derived from petrography and major element fractionation is modeledas somecombination of Rayleigh chemistry. and equilibrium processes (the latter produces shorter vectors at9lVofractionation). Although a small amount of Parental magma and fractionation path biotite fractionation is permitted from trace element data, In order to understand the fractionation scheme that it seems unlikely because all biotite in the gabbro is relates the Concord gabbro and syenite, it is important to postcumulusand syenite contans very little biotite. Thus estimate the composition of the parental magma. The

1d (o)

10i v,

Gobbo

o Gobbro * 3yonite O Avoro3roGobbro fAvrroge Syrnite Fig' ll. (a) Rb-Sr and (b) Rb-Zr variation diagrams (ppm) for individual samplesand averagevalues of Concord gabbro and syenite. The vector diagramsto the right show the efects of crystallizing the assemblageplagioclase + olivine + clinopyroxene * magnetitein the proportions indicated by the petrologic mixing program(Tabte 6), as well as the effectsofother possiblefractionating phases.Both Rayleigh and equilibrium fractionation trends, calculatedusing the equationsofGast (1968)and Greenland(1970), are illustrated for the proposedfractionating assemblage.Distribution coefficientsfor Rb, Sr, and Zr for various phasesdepend on liquid composition;for these vector calculationsan averagevalue for silica (55Vo)intermediate between that for averagegabbro (487a)and syenite (61%) was chosen. These values (Arth, 1976;Cox et al., 1979;Pearceand Norry, 1979)are as follows: amph biot mt plae cpx olv opx kspar KBb o.l5 3.26 0 0.48 0.001 0.001 0.022 0.34 KBI o.zs 0.12 0 2.84 0.29 0.001 0.017 3.87 KBI l.oo 1.20 0.20 0.015 0.14 0 0.05 0 OLSEN ET AL.: CONCORD GABBRO-SYENITE COMPLEX 331

Concord gabbro samples almost certainly represent cumulate rocks (albeit containing significant postcumulus material), so it is unlikely that whole rock analyses precisely representliquid compositions.This can be seen o in Figure 12, in which calculated norms of 45 analyzed 9ro gabbro samples(data from Cabaup, 1969and Butler and + o Ragland, 1969) do not fall within the circled fields of 0 25 either alkaline or tholeiitic basalts. Fractionation of olivine * plagioclase * clinopyroxene will displace these samples relative to the Ol-Di join in this figure, along which presently cluster. Therefore, it Stoz many of them . concord Gobbro seems likely that the parental liquid must have been t C*ord SlFnil. tholeiitic, although picritic or transitional basalt composi- Fig. 13.Alkalis versussilica (wt.%) for Concordgabbro and tions cannot be ruled out. syenitesamples. The projectionof the planeof critical silica Morse (1980), in a discussion of what he termed the undersaturationis representedby thedashed line (Ol-Cpx-Plat "syenite problem", questioned the ability of a basaltic the basalticend, Ab-Or at the trachyticend, after Cox et al., of this liquid with composition even slightly deviant from the 1979).Basalts with compositionslying close to eitherside (schematicallyrepresented by largedots) may fractionate to critical plane of silica undersaturation(FeDi-Ab) in the line producetrachyte before "falling off' thisthermal divide towards normative basalt tetrahedron to produce a trachytic (sy- thephonolite or rhyoliteminima. enitic) liquid. In the system NaAlSiO+-KAlSiOa-SiO2 at low pressure, a basalt of critical plane composition will fractionate to a syenitic liquid near the feldspar minimum, which is actually a narrow thermal maximum relative to possible that the thermal maximum in this simple experi- the nepheline and silica minima. Morse suggestedthat a mental system may disappear at high pressure or when slight deviation from the critical plane composition additional componentsare added. should causethe liquid to migrate down one of the steep The absenceof any rocks of intermediate composition liquidus slopes to either side of this thermal maximum, indicates that fractionation to form the syenitic liquid producing phonolitic or rhyolitic residual liquids, respec- occurred at depth. Similar bimodal distributions are com- tively. However, Cox et al. (1979) argued that during mon in extrusivebasalt-trachyte provinces (e.g', Upton, fractionation of basalts lying slightly to either side of the 1974).One possible explanation for the missing interme- thermal divide, the residual liquids are sufficiently en- diate rocks is that intermediate liquids may be more riched in alkalis to become trachytic long before the viscous, i.e., less mobile, than basaltic and syenitic ultimate phonolitic or rhyolitic residues can be produced liquids (Upton, 1974).Another possibility is that a fairly (Fig. l3). This appears to support the suggestion by flat portion ofthe liquidus trajectory between gabbro and Morse (1980)that the feldsparjoin ofpetrogeny's residual syenite at high pressure might cause a large degree of system may be more accurately representedby a thermal fractionation over a small temperature interval, so that plateau than by a sharply defined saddle. It is also not many rocks of intermediate composition would be produced.

Conclusions (l) Geophysical modeling suggeststhe Concord complex consistsofa 6 km-deepgabbro pluton surroundedby a steep-walledsyenite ring dike, dipping outwardly at an angle of about 78' on the west side, with a vertical dip on the east side. This configuration probably resulted when the syenitic liquid intruded the fractured contact region between the gabbro body and the surrounding country rock. (2) Gabbro and syenite crystallized nearly contempora- t+311671e4196 neously at 405 m.y. ago. Initial ETSr/86Srun4 showing Fig. 12. Expanded normative basalt tetrahedron ratios are consistent with the interpretation that both calculated values for znalyzed Concord gabbros. Normative lithologies are related through igneousfractionation, and calculationsare tabulatedin Olsen(19E2). Most compositionsare materials is excluded. tholeiitic, but many cluster about the Di-Ol join, the boundary significant assimilation of crustal between the tholeiitic and alkali basalt fields. Circled areas are (3) Petrologic observations and geochemicalmodeling compositional limits for flows (magma compositions) from suggestthat production of the Concord syenite involved typical basalt provinces. extensivefractional crystallization of olivine, plagioclase, 332 OLSEN ET AL,: CONCORD GABBRO_SYENITE COMPLEX

and clinopyroxene, with minor oxide and sulfide miner- Gast, P. W. (1968)Trace elementfractionation and the origin of als, from a basaltic parental magma similar in composi- tholeiitic and alkaline magmatypes. Geochimicaet Cosmochi- tion to the Concord gabbro. This fractionation occurred mica Acta, 32, 1057-1086. (1970) at depth before intrusion of the syenitic magma to the Greenland, L. P. An equation for trace-elementdistribu- present level. tion during magmatic crystallization. American Mineralogist, 55.455465. (4) The Concord trend probably reflects differentiation Irvine, T. N. (1982)Terminology for layered intrusions. Joumal of a tholeiitic basaltic liquid to a syenitic residual liquid of Petrology, 23, 12'1-162. along a path near the critical plane of silica undersatura- Johnson, R. W. and Bates, R. G. (1960) Aeromagnetic and tion. This trajectory may represent a thermal ridge of aeroradioactivity survey of the Concord Quadrangle, North sufficient width to prevent fractionating melts from de- Carolina. U.S. Geological Survey Professional Paper 4008, scending toward the rhyolite or phonolite minima before lE2-195. reaching syenitic compositions, or the thermal maximum Jones,L. M. and Walker, R. L. (1973)Rb-Sr whole rock age of in the analogous experimental system may have been the Siloam granite, Georgia: A Permian intrusive in the southern modified by the effects of pressure or additional compo- Appalachians.Geological Society of America Bulle- nents. tin.84. 3653-3658. Leake, B. E. (1978) Nomenclature of amphiboles. American Acknowledgments Mineralogist, 63, 1023-1052. LeGrand, H. E. and Mundorff, M. J. (1952)Geology and ground grateful We are to the following individuals who have greatly water in the Charlotte area, North Carolina. North Carolina aided this study: R. H. Hunter for assistancein geochemical Department of Conservation and Development, Bulletin 63. modeling,K. C. Misra, L. A. Taylor, R. H. Hunter, J. R. Butler, McCallum, I. S., Raedeke,L. D. and Mathez, E. A. (1980) and T. L. Wright for critical review; J. R. Butler for generously Investigations of the Stillwater complex: Part I: stratigraphy sharingchemical data; S. R. Haxt for making facilities available and structure of the banded zone. Americal Journal of Sci- for isotopic analyses;R. A. Hopkins for assistancewith gravity ence,2804, 59-87. modeling; and T. L. Grove for thoughtful discussion. Isotopic McSween, H. Y. (1980)Mineralogy and petrology of the Dutch- work was supportedby NSF Grant EAR-7803342.Field work by mans Creek gabbroic intrusion, South Carolina: reply. Ameri- the senior author was supportedby grantsfrom SigmaXi and the can Mineralogist,65, 1304-1306. Geological Sciences Frofessors' Honor Fund, University of Medlin, J. H. (1968) Comparative petrology of two igneous Tennessee. complexes in the South Carolina Piedmont. Unpublished Ph.D. Thesis,Pennsylvania State University. References Morgan, B. A. and Mann, V. I. (1964)Gravity studiesin the Albee, A. L. and Ray, L. (1970)Correction factors for electron Concord Quadrangle,North Carolina. SoutheasternGeology, probe microanalysis of silicates, oxides, carbonates, phos- 5, 143-155. phates, and sulfates.Analytical Chemistry, 42, 1408-1414. Morse, S. A. (1980) Basalts and Phase Diagrams. Springer- Amber, E. P. and Ashley, P. M. (1980)Mineralogy and petrology Verlag, New York. of the Dutchmans Creek gabbroic intrusion, South Carolina: Neilson, M. J. and Brockman, G . F . (1977)The error associated discussion.American Mineralogist, 65, 1302-1303. with point counting. American Mineralogist, 62, 1238-12M. futh, J. G. (1976)Behavior of trace elements during magmatic Nockolds, S. R. (1954)Average chemical compositionsof some processes.---asummary of theoreticalmodels and their applica- igneous rocks. Geological Society of America Bulletin, 65, tions. U.S. GeologicalSurvey Journal of Research,4,4147. 1ffi7-1932. Bell, H. (1960) A synthesis of geologic work in the Concord Olsen,B. A. (19E2)Petrogenesis ofthe Concordgabbro-syenite Area, North Carolina, U.S. Geological Survey Professional complex, Cabamrs County, North Carolina. Unpublished Paper4fi)8, 189-191. M.S. Thesis,University of Tennesseeat Knoxville. Bence, A. E. and Albee, A. L. (1%8) Empirical correction Pearce,J. A. and Norry, M. I. (1979)Petrogenetic implications factors for the electron microanalysisof silicates and oxides. of Ti, Zr, Y, and Nb variations in volcanic rocks. Contribu- Journal of Geology, 76,382403. tions to Mineralogy and Petrology, 69, 3347. Butler, J. R. and Ragland, P. C. (1969)A petrochemical survey Price, V. (1969)Distribution of trace elementsin plutonic rocks of plutonic intrusions in the Piedmont, SoutheasternAppala- of the southeastern Piedmont. Unpublished Ph.D. Thesis, chians, U.S.A. Contributions to Mineralogy and Petrology, University of North Carolina at Chapel Hill. 24, t&-t90. Sando,T. W. and Hart, S. R. (1982)Nd isotope geochemistryof Cabaup, J. (1969)Origin and difierentiation ofthe gabbro in the Hercynian granitic rocks of the southeasternAppalachians. Concord ring dike, North Carolina Piedmont. Unpublished Geological Society of America abstracts with programs, 14, M.S. Thesis, University of North Carolina at Chapel Hill. 79. Cox, K. G., Bell, J. D. and Pankhurst, R. J. (1970) The Sneeringer,M. A. (1981) Strontium and samarium diffusion in Interpretation of Igneous Rocks. Allen and Unwin, London. diopside. Unpublished Ph.D. Thesis, MassachusettsInstitute Fullagar, P. D. (1971)Age and origin of plutonic intrusions in the of Technology. Piedmontof the southeasternAppalachians. Geological Socie- Streckeisen, A. L. (1973) Plutonic and ty of America Bulletin, 82,2845-2862. nomenclature.Geotimes, 18, 43-63. Fullagar, P. D. and Butler, J. R. (1979)320 to 2ffi Myr-old post- Talwani, M., Worzel, J. L. and Landisman, M. (1959) Rapid metamorphicgranitic plutons in the Piedmontof the southeast- gravity computationsfor two-dimensionalbodies with applica- ernAppalachians.AmericanJournalofScience,2Tg.16l-185. tion to the Mendocino submarine fracture zone. Journal of OLSEN ET AL.: CONCORD GABBRO-SYENITE COMPLEX 333

GeophysicalResearch, g, 49-59. in the Carolinas, p. 28-37, Carolina Geological Society Field Upton, B. G. J. (1974) The alkaline province of south-west Trip Guidebook, S. C. Geological Survey, Columbia. Greenland.In H. Sorensen,Ed., The Alkaline Rocks,p. 221- Wright, T. L. and Doherty, P. C., (1970)A linear programming 23E,Wiley and Sons, London. and least squares computer method for solving petrologic Van der Plas, L. and Tobi, A. C. (1965)A chart for judging the mixing problems.Geological Society of American Bulletin' 81, reliability of point counting results. American Journal of r95-2008. Science,263, 87-90. York, D. (1966)Least-squares fitting of a straight line. Canadian Watson, T. L. and Laney, F. B. (1906) The building and Journalof Physics,44, 1079-1086. ornamentalstones of North Carolina. North Carolina Geologi- Zindler,A., Hart, S. R., Frey, F. A. and Jakobsson,S. P. (1979) cal Survey Bulletin, 2. Nd and Sr isotope ratios and rare-earthelement abundances in White, W. A. and Mundortr, M. J. (1944)Unpublished geologic ReykjanesPeninsular basalts: evidence for mantle heterogene- map of the Concord area, North Carolina. ity beneath lceland. Earth and Planetary ScienceLetters, 45, Wilson, F. A. (1981)Geologic interpretation of geophysicaldata 249-262. from the "Mecklenburg-Weddington" gabbro complex, southern Mecklenburg County, North Carolina. In J. W. Horton, J., J. R. Butler and D. M. Milton, Eds., Geological Manuscript received, March 16, 1982; Investigationsof the Kings Mountain Belt and Adjacent Areas acceptedforpublication, October 13, 1982.