Axinite Mineral Group in Low-Grade Regionally Metamorphosed Rocks In

AmericanMineralogist, Volume 65, pages 1119-1129, 1980

Axinite mineral group in low-graderegionally metamorphosedrocks in southem New Zealand

leN J. PnrNcre AND YosuKE KAwAcHI Geology Department, Univerity of Otago Dunedin, New Zealand

Abstract

In southern New Zealand, minerals of the axinite group are widespread in vein assemblages in regionally metamorphosed rocks of the prehnite-pumpellyite, pumpellyite-actinolite, and chlorite zone greenschistfacies. Fe, Mg-axinites, approachingend-member ferroaxinite in composition, along with quartz and often prehnite, pumpellyite, iron-rich epidote, and chlorite fll veins in spilitized volcanic and greywacke lithologies. Tinzenite and more cofilmonly manganaxinites occur in quartz veins in nearby femrginous and manganiferous cherts. Chemical analysesof vein axinites are presented,as w€ll as analysesof four porphyroblastic ferroan manganaxinites which occur as rock-forming minerals at widely sep- arated localities. Compositional variability in published axinite analysesalong with those of this study can be attributed partly to formation temperatures.A low-temperature miscibility gap may exist in the axinite group. Tinzsnile or manganaxinite and ferroaxinite are stable in low-grade metamorphic rocks of appropriate compositions, whereas ferroan manganaxinites and man- ganonn ferroaxinites occur in some pegmatites, skarns, and regionally metamorphosedrocks which equilibrated at more elevated temperatures. For a wide lange of burk compositionsin biotite and garnet zone greenschistfacies and amphibolite facies rocks of southern New Zealand, tourmaline is the only observedborosili- cate phase. At metamorphic conditions typical of these grades,axinite minerals would be restricted to relatively Ca-rich lithologies by a reaction of the form: 3 ferroaxinite + 2 chlorite + 2 albite * 5 quartz : 2 tourmaline * 4 epidote + 2 actinolite * 5 water

Introduction spread as being mainly due to host rock composition. The general formula for the axinite group pro- Minerals of the axinite group have long been rec- posedby Lumpkin and Ribbe (1979)is: ognizedas typical phasesin manganese-oredeposits, skarns, and pegmatitesand associatedhigh-temper- [(Mn,Fe'*,M g,Zn Al)(Car-,Mn ) (Alz-,Fef*)]]I atureenyironments. Ozaki (1970,1972\ examined the (oH,_-o-x(Brs*--Al,)I'o3ol relationship between mode of occurrence of axinite and chemical composition. He distinguished five dif- where w < 1, r ( l, y 11 l, z 11 l, and VI and IV ferent categories:manganese and femrginous ore de- represent co-ordination of the cations. Sanero posits,pegmatites, contact metamorphic and metaso- and Gottardi (1968) have clarifed the nomen- matic ore deposits, regionally metamorphosedrocks, clature and defined the end members ferroaxinite and veins in igneous and sedimentary rocks. When [Fe2*CarAlrBSioO,r(OH)]and manganaxinite axinite analysesfrom each occurrence are plotted on [Mn'*CarAlrBSLO,r(OH)]. The name ferroaxinite is a Mn-Ca-Fe diagram the manganesecontents of ax- applied to thoseaxinites with Ca > 1.5and Fe ) Mn, inites in each rock type are shown to decreasein the and manganaxinitewhere Ca > 1.5 and Mn > Fe. above order, although considerable overlap is ob- Tinzenite includes those axinites with Ca < 1.5 and served(Fig. l). Ozaki interpreted this compositional Mn > Fe which approach the empirical composition ffi03-.00/.x/80/ I r 12-l l 19$02.00 ll19 I 120 PRINGLE AND KAWACHI: AXINITE

l. manganese and ferrugrnous ore deposits.

2. pegmatites

3. contact netamorphic and metasomatic ore dePosits

4, regional metamorphic rocks

5. veins in igneous and sedinentary rocks.

30 lln Irc / \

Fig. l. Triangular Mn-Ca-Fe diagram after Ozaki (1972), showing compositional variations of axinites in different [thologies.

MnrCaAl,BSLO,s(OH).Sanero and Gottardi (1968), records of axinite from veins in regionally metamor- followed by Ozaki (1969, 1972) and Lumpkin 6d phosed rocks of grads higher than zeolite facies have Ribbe (1979), showed that substitution in the group been reported. The majority of these are in rocks is principally in two series, one from ferroaxinite to 6pafaining mineral assemblagescharacteristic of the manganaxinite, the other fron manganildnite to- amphibolite facies and lower grade. wards tinzenite. Recent workers on low-grade metamorphic rocks Axinite in low-grade regionally metamorphosed in southernNew Zealand have recognizedminerals of rocks was first reported by Kojima (194) in the the axinite group at various localities in several dis- Sambagawametamorphic belt in central Japan. Here tinct terranes(Coombs et al., 1976)flanking the axis axinite occurs in a stilpnomelane-bearing band in of the Haast Schist(e.9. Mason, 1959;Mansergh and greenschist.The only extended account ofaxinite as Watters, 1970;Read and Reay, l97l; Andrews et al., a fock-forming mineral in schistsis by Nureki (1967), 1974; this study) (Fig. 2). Four occurrenoesare as who described and partially arralyzedmanganaxinite rock-forming minerals, others are confined to veins, from transitional blueschist-greenschistfacies rocks but all are restricted to rocks of the chlorite zone of the Sangunmetamorphiczonein soutlwest Japan, gteenschist facies, pumpellyite-actinotte facies, and where it occurs as inclusion-studded porphyroblasts prehnite-pumFellyite facies. We suspectthat such and as clear grains in nearby quartz-albite-ildnite occurrencesare relatively abundant and that miner- veins. The manganaxinite-bearing assemblagescon- als of the axinite group are more coilrmon in region- tain stilpnomelane * chlorite + epidote + albite + ally metamorphosed sequencesthan has generally qrraftz + muscovite + calcite + sphene + opaque. been appreciated. Examples of axinite-bearing veins in regionally metamorphic rocks are more numerous. Simonen Field relations and petrography and Wiik (1952) describe ferroaxinite-quartz-calcite veins in amphibolites and basic intrusive rocks in Torlesseterrane and adjacentHaast Schistterrane Finland. In a review of axinite oocurrencesin the These terranes consist of Late Paleozoic to Meso- Norwegian Caledonides,Carstens (1965) emphasized zoic greywackes and argillites together with'rare the association of epigenetic axinite-bearing veins cherts, spilitized pillow basalts, limestones and con- with metabasite lithologies. Worldwide, over forty glomerates, and their schistose derivatives. Axinite PRINGLE AND I(AWACHI: AXINITE tl2l

Fig. 2. Axinite localities in the southern half of the South Islant axinite occurrences:(a) Lord Range(Andrews et al-,1914);(b) Pertl (d) Humboldt Mountains (Bishop et al.,1976)-Dots are localitiesof PareoraGorge (OU a672); (3) Kirkliston Mountains (OU 33912) 35524,15597, 35526, 35523, 35528); (5) Hawkdun Range (Gradv, 35595,35596,28558,33366,28n$;Q) Akatore(Read and Reay,l97l) (OU 25686)'

hema- minerals along with other low-grade metamorphic matic tourmaline, manganoanstilpnomelane, phasesoccur in veins at numerous locatties. Axinite tite, and abundant calcite' porphyroblasts, however, are more restricted in dis- At lower grade, in pumpellyite-actinolite facies man- tribution. rocks, veins containing quartz and pale brown Near Dansey Pass, thick axinite-bearing quaftz ganaxinite (OU 35527) occur in massive hematitic veins cut a schistosemetachert in chlorite zone green- metacherts. Pinkish brown and pale purple ferroaxi- veins schist facies rocks of textural zone IIIA (Bishop, nites are cofilmon constituents of anastomosing 35524' 1972). TtLe axinite, a manganaxirdte (OU 25336),' in nearby metabasitelithologies (OU 35523' in thick- coexists with hematite and manganoan brunsvigite 35525,35526, 35597). The latter veins vary in ad- which contains 4.04.3 weight peraent MnO. Minor nessfrom a fe* mm to severalcm and contain, of cal- tourmaline, along with spessartineand muscovite, is dition to ubiquitous quattz, variable amounts present in the host rock but not in the axinite-bearing cite and iron-rich epidote (Psru-Psr.).lron-rich cm veins. Elsewherein the chlorite zone, subhedral man- epidoteis often concentratedas border zones'l-3 ganaxinite porphyroblasts(OU 35528)studded with wide in thicker ferroaxinite-bearing veins. Albite, minute inclusions predominantly of quartz and hem- pumpellyite, chlorite, asbestiform actinolite, sericite, phases' atite occur within stilpnomelane-chlorite-epidote la- pyrite, and sphene are common accessory minas in a fine-grainedhematitic metachert.Also, an Mg-pumpellyite containing 2.5 weight percent MgO impure marble contains small grains of ferroan man- coexistswith ferroaxinite in OU 35526. ganaxinite (OU 25335) mantled by manganoan Manganaxinites in veins from two other localities 70 km brunsvigite. Other phases in this rock include pris- were analyzed. In the northern Malvern Hills, west of Christchurch, buff-brown manganaxinite #1676)fills thin veinlets along with quartz in a I numbers are Geology Department, UniversitY of (OU Sample metachert. The host Otago catalogue numbers. rhodonite-pyrolusite-bearing tr22 PRINGLE AND KAIIACHI: AXINITE

rock forms part of a 600-m-thick sequenceof me_ inclusions of quartz and hematite which causea pink tabasites, hematitic and manganiferous metacherts, to pale brown coloration of cores in some grains and marbles metamorphosed to prehnite_pump_ (Fig. 3). ellyite facies. The Torlesse rocks pass gradationally into the A large boulder of pumpellyite-bearing metachert Haast Schist terrane with an increasing development in river gravels of Station Stream, which drains the of penetrative schistosity. Metamorphic grade in eastern Kirkliston Mountains, contains veins up to these transitional areas is commonly of pumpellyite_ several cm thick of quartz and yellow-brown man_ actinolite facies. It is near the marginal fringe of the ganaxinite (OU 33912). Red and green metacherts Haast Schist terrane that axinite-bearing assemblages are important constituents in a metavolcanic se_ are widespread. Axinite has not been reported from quenc€ cropping out nearby, and probably the boul_ schists of biotite zone or of highsl metamorphic der is locally derived. grades in New Zealand. Other analyzed axinites from vein assemblagesin Torlesserocks are from tl.e Hawkdun Range (Grady, Caplesterrane 1968), where pale brown ferroaxinite (OU 21067, The CaplesGroup flanking the southwesternmar- 21069, 21070) along with quartz and prehnite oc_ gin of the Haast Schist terrane consistslargely of vol- cupiesveins cutting prehnite-pumpellyite facies me_ canic arc-derived sediments with minor spilitic vol- tagreywackes. Axinite-bearing veins in similar lith_ canics,limestones, and cherts.These rocks form part ologies have been perth described from the River of the somplexly deformed Te Anau Assemblageof (Mason, 1959)and from near Lake Aviemore (Man_ Turnbull (1977). sergh and Watters, 1970). These samplesdiffer from Axinite-bearing qluartz veins occur widely near others in this study in fsing remote from metabasite Lake Wakatipu (e.g. Kawachi, 1975; BishoB et aI., and,/or cherty lithologies. 1976).Fenoaxinites from several localities were ana- Porphyroblastic manganaxinites occur together lyzed. In each casethey are from veins in mafic vol- with qlartz, ferrostilpnomelane,minor manganoan canogenic rocks of prehnite-pumpellyite or pump- ripidolite, and sericite in a prehnite-pumpellyite ellyite-actinolite facies. Associated minerals, as in facies metachert pareora (OU 4672) from the Lower the ferroaxinite-bearing assemblagesof the Torlesse Gorge (Fig. 2). The porphyroblasts comprise six per- terrane, include qu,afiz, pumpellyite, epidote, chlo- cent of the rock, are typically wedge-shapedcrystals rite, calcite, and albite. up to 300 pm diameter, and are studdedwith minute A Mg-pumpellyite coexisting with ferroaxinite in

Fig' 3' Manganaxinitc porphyroblasts in oU 4672. Minute inclusions of quartz and hematite ar€ concentrated in grain cores. The fne-grained groundmass consistsof quart4 ferrostilpnomelane, manganoan aliOotite, and minor spessartine.Scale bar is 250 pm. PRINGLE AND KAWACHI: AXINITE tt23

OU 28558 contains 3.1 weight percent FeO*' and necessary.Using this method most analyzedferroaxi- 3.40 weight percent MgO, and is typical of pump- nites have no or low FerO, content (Table 2). How- ellyite compositions in rocks of the higher-grade ever, for most manganaxinitesFerO. content repre- parts of the pumpellyite-actinolite facies (Kawachi, sents a significant amount of the total iron. Likewise, 1975).The composition of coexistingplagioclase is wet-chemically determined compositionsof manga- almost end-memberalbite (Ab", ') and calcite in the naxinites listed by Ozaki generally have higher same vein is close to pure CaCOr. Optical data for Fe'*: Fe'* ratios than ferroaxinites (Ozakt, 1972). these phasesin other ferroaxinite-bearingvein as- MnO contents of manganaxinitesin Table I range semblagessuggest similar compositions.Epidotes are from 7.19 to 16.25weight percent and FeO * FerO, all iron-rich, as indicated by their high birefringence rangesfron 4.29 to 0.48 weight percent.Total (Fe + and yellow to pale greenpleochroism. Mn + Mg) of ferroaxinites closely approaches2.00 Although ferroan manganaxinite porphyroblasts per formula unit and there is a negative correlation occur in pale greensericitic schist(OU 33366)associ- betweentotal Fe and Mg, an oppositetrend to Fe- ated with pillowed metabasaltsnear Mt. Nicholas in Mg variations seenin analyzedmanganaxinites (Fig. the Thomson Mountains (Turnbull, 1974),as yet no 4a). With increasingMg content,Mn in manganese- manganaxinites have been identified in veins in this rich phasesdecreases rapidly from tinzenite(Fig. ab). terrane. The ferroan manganaxinite porphyroblasts Ferroaxinites, however, show limited Mg-Mn sub- comprise5.5 volume percentof the rock and are dotted stitution, despitehigher and more variable Mg con- with minute inclusions mainly of quartz and albite. tent. Ferroaxinite analyses plot within a restricted The crystals are com-monlyidioblastic lozenge-shaped field near the Ca-(Fe + Mg) join on a Ca-Mn-Fe tri- grains less than 200 pm in maximum dimension and angular diagram (Fig. 5). They fall within the com- are rarely twinned. Pumpellyite, along with quartz, positional range of axinitesfrom other low-grade,re- phengite, albite, chlorite, sphene,and minor detrital gionally metamorphosedterranes (compare Fig. l) epidote,completes the mineral assemblage. and closely approach the ideal formula of ferroaxi- Along strike to the southeast, on the coast, com- ilte. parable rocks known as the Tuapeka Group also Tinzenite from Akatore (Table 1,25686\ contains gradenorthwards into the Haast Schistterrane. Here, considerablymore MnO and lessCaO than our ana- near Akatore Creek, yellowish tinzenite (OU 25686) lyzed manganaxinites and approachestheoretical tin- occupies thin veinlets along with quartz, rhodonite, zenite composition. Although magnesium contents pyroxmangite, and apatite in an akatoreite-bearing are negligible,some of the analysesare calculatedto manganiferousmetachert (Read and Reay, l97l). contain up to 0.33 weight percentFerOr. In contrast, tinzenite ftsrr finzens, Switzerlandhas 1.59weight Mineral chemistry of tinzenite and axinites percentFerO, (Milton et al.,1953). Major elementsexcept for boron and water have Porphyroblastic manganaxinite crystals from the been analyzedby a JEoLJxA-5A computer-controlled Torlesse terrane in the Lower Pareora Gorge are electron microprobe. Analysis conditions, standards, compositionally zoned (Fig. 2), and minute quartz and correction procedure are similar to those de- and hematite inclusions (

Table l. Average and selectedanalyses of tinzenite a1d manganaxinites

ti nzenite Specimen 0u25686-0uzs336 0u44676ou33er2 0u35527 0il3!31['-tlJliiru 0U44672 0U33366 Numberof .NFA rtm analyses averaged 76 E] (b)ll

si02 43.3 42.6 42.5 42.7 42.2 43.6 42.0 46.3 42.6 +z-t 42.0 ri02 0.02 0.07 0.03 0.05 0.01 0.00 0.04 0.17 0.18 0.01 0.1r At ^0- 18.2 I 6.9 ]6.5 17.1 17.s 16.6 ]6.9 15.9 (a) 17.3 t 8.6 I8.1 C^ n 0.l0 'l.96 .J 0.74 1.23 0.45 2.18 I .64 (a) 0.74 0.00 0.00 FeO 0.09 0.00 0.75 0.74 0.65 0.32 2.06 2.44(") I .99 ? lo 4.29 Mn0 20.6 13.12 13.12 13.14 11.72 11.37 ll.31 10.53 11.44 8.78 7.19 Mgo 0.08 0.30 0.1 9 0.14 0.30 0.46 0.62 0.18 0.20 n?? u. f,o Ca0 ]3.3 18.9 17.6 17.9 18.8 18.3 18.6 16.4 17.8 t9.5 19.2 Na20 0.03 0.03 0.02 0.03 0.02 0.03 0.02 0.04 0.04 0.01 0.05 Kzo 0.00 0.01 0.03 0.02 0.02 0.03 0.01 0.04 0.04 0.05 0.08 Total 95.72 92.67 92.70 93.05 91.67 92.89 93.20 92. 00 YZ. J5 o? 17 9l .58

"2"3 6.24 6.il 6.07 6.12 6.07 6.14 6.10 6.t0 6.20 6.10 'I Hr0 (a) .61 'l.57 1.58 1.57 1.58 1.59 t.?6 l.s8 I.60 1.58

Atomic proportions, 0 = 28

si 8.04 I .08 8.12 8.09 o.u3 8.23 70q 8.09 7.98 7.98 Ti 0.00 0.0t 0.00 0.01 0.00 0.00 0.0t 0.03 0.00 0.02 AI 3.98 3.78 3.71 3.82 J. YJ 3.69 3.79 3.87 4.l0 4.05 Fe-' 0.02 0.1I 0.28 0.t 8 0.06 0.31 0.23 0.I| 0.00 0.00 FE 0.01 0.00 0.12 0.12 0.10 0.05 U. JJ 0.32 0.50 0.68 ,l.82 .l.82 'l.84 Mn 5 .2+ 211 2.12 2.11 lao 1.39 l.16 Mn 0.02 0.09 0.05 0.04 0.09 0.13 0.l8 0.06 0.09 0.16 z.05 3.84 J.OU J.bJ 3.84 3.70 ? 70 ?A? ?On ?Ol Na 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.02 K 0.00 0.00 0.01 0.01 0.01 0.01 0.00 0.0r 0.01 0.02 total 17 07 I 8.03 18.02 18.02 17.98 I7.95 I8.l4 17.97 17.98 I8.00 Analyst YK IJP IJP IJP IJP IJP IJP IJP IJP IJP

(a) Calculated(see text). (b) 44672(core) recalculatedassuming an excessof 3.70%Si0r. (c) Total Fe as FeO.

aligned subparallel to schistosity, are also composi- samples to several weight percent MnO in chlorites tionally unzoned.The Fe: Mn ratio of a typical anal- associatedwith manganaxinite.Ranges for phasesin ysis (Table l, OU 33366),however, is slightly higher five samplesare listed in Table 3. than Fe: Mn ratios of porphyroblastic axinites north- east of the Haast Schist axis. Discussion In general, porphyroblastic axinite compositions are intermediate between m'nganaxinites and fer- Axinite compositionand metamorphicgrade roaxinites from nearby metamorphic vein assem- In southern New Zealand the common association blages.This relation5hiFis shown in Figure 5. of Mg-ptrmFellyite with axinite minerals and the re- Mn distribution between coexisting chlorite and stricted occurrence of axinite to rocks of low meta- axinite varies consi$tently from negligible and minor morphic rank suggest that temFeratures and pres- MnO contents in chlorites frorn ferroaxinite-bearing sures of axinite formation are unlikelv to have PRINGLE AND I(AWACHI: AXINITE tr25

Table 2. Average and selectedanalyses of ferroaxinites

0U2l067 0u2l069 0u355260U28558 Jts!v |,,,!, I 0u355250U2l 070 0u355230U35524 0u355960U35s97 0u28734 Numberof 1 t4 44 JJ 7 l0 447 analyses averaged 43.2 si02 +J. J 43.0 42.5 42.6 43.0 42.7 42.1 42.9 42.8 42.4 42.2 Ti02 0.02 0.00 0.02 0.04 0.0] 0.02 0.0] 0.05 0.03 0.01 0.0] 0.04 'I '18.4 Al ^0^ 8.0 18.2 I I .9 18.6 18.8 17.s 18.3 18.4 18.5 18.7 17.9 ZJ (") Fe^o^ 0.28 0.00 0.00 0.00 0.00 0.75 0.00 0.00 0.00 0.00 0.00 0.46 a5 (") Feo 8.37 6.87 6.]5 8.24 I .96 8.9] 8.78 9.26 9.61 7.55 ,l.08 Mn0 I .95 1.58 l.5l t -zt | .zo 1.25 l.ll 0.98 0.72 0.59 0.46 '| 'l.33 'I Mso lE 1.97 l.l9 tqE 2a1 l 39 .50 ].36 L20 1.21 2.40 '19.6 Ca0 20.1 20.0 20.2 20.1 20.0 20.3 20.0 20.1 20.1 20.1 20.1 Naro 0.02 0.02 nd nd nd 0.04 0.01 0.02 0.03 nd nd 0.03 Kzo 0.01 0.02 nd nd n0 0.0] 0.01 0.01 0.02 nd nd 0'02 TotaI 92.70 92.14 92.45 92.13 92.23 9r.90 92.13 92.57 92.50 92.19 92.42 92.16

''' 6.]8 6.22 B^0^ o,al 6.21 6.20 D.az o.40 6.14 6.ll h.al o.4l 6.17 a5 'I 'L59 'l.59 1.60 1.61 (a) t.ol I .6r l.6l .61 1.62 I .61 l.6l t.60 H^0z

Atomicproportions, 0 = 28 si 8.088.027.g47.g37.s68.067.s37.987.997'967'918'05

tl 0.000.000.000.0]0.000.000.000.0]0.000.000.000.0.1 AI 3.964.004.164.084.]03'894.064.044.054.094'133'93 ?+ Fe'' 0.040.000'000'000.000.,1.10.000.000.000.000.000.07

fe 1.3ll.l3l'30].070.951.30].41].39].371.451'5]1..18 0'12 0'09 0'07 Mn 0.3] 0.25 0.24 0.20 0.20 0.20 0.]8 0.]7 0']6 Mg 0.320.550.330.7]0.800.390.370.420.380.340.340.67

LA 3.g24.024.004.033.984'044.093.994.024.044.044.01 0'01 Na 0.ol 0.01 0'02 0.00 o'0] 0'ol 0'01 K 0.00 0.01 0.00 0.00 0.00 0'01 '17.95 18.00 18.02 18.01 TotaI 17.gg lt.s7 18.03 17.gg ]8.01 18.04 18.0] 17.99

Analyst IJP IJP IJP ]JP IJP YK YK YK YK,IJP IJP IJP YK

(a) Calculated (see text). nd = not determined. exceededthe upper stability limit of Mg-pumpellyite. pumpellyite-actinolite facies, whereas rocks of cate- Recent experimental work by Schiffman and Liou gory 4 have been regionally metamorphosed to (1977) and Plyusina and Ivanov (1978) have estab- greenschist and lower amphibolite facies. Similarly' lished the upper stability limit of Mg-pumpellyite to manganeseand femrginous ore deposits soil4ining between340" and 390'C over a P(fluid) rangeof I to manganaxinite or tinzenite (category I of Ozaki, 8 kbar. Temperatures well in excess of these un- 1972\ have been affected by low-grade Sambagawa doubtedly prevailed during axinite formation in metamorphism. Axinites in rocks sf highsl-fs6ps1- many pegmatites and skarns reported elsewhere(e.9. ature paragenesessuch as skarns and pegmatites are Mozgova, 1964). commonly ferroan manganaxinites or manganoan Original locality descriptions of the analyzed ax- ferroaxinites intermediate in composition. between inites listed by Ozaki (1972), together with our data, ferroaxinite and tinzenite end members. In general, reveal that the composition of axinites in regionally ferroaxinite and tinzenite are stable in low-grade metamorphosed terranes varies systematically with rocks but become more Mn- and Fe-rich respectively metamorphic grade. The host lithologies of Ozaki's with increasing metamorphic grade. category 5 (Fig. l) invariably contain mineral assem- This trend is compatible with our observations. blages typical of the prehnite-pumpellyite and Porphyroblastic manganaxinites have compositions t126 PRINGLE AND KAWACHI: AXINITE

detectable in porphyroblastic axinites n OU 4672 may reflect axinite growth accompanying increasing metamorphism. Compositions of grain cores plot near the field of vein nanganaxinites on the Ca-Mn- (Fe + Mg) triangular diagram (Fig. 5). However,the distinctly FeO*-, MgO-, and CaO-enriched and MnO-depleted rims of the samegrains are more oen- trally located on the figure. Banno and Matsui (1979,p.68) showed that the stability temperatures of intermediate members of any mineral solid solution series depend on differ- encesin ionic radii of substitut:ngions. From Banno and Matsui's data, differences in ionic radii between the divalent ions Mn, Fe, Mg, Mn, Ca suggestthat in 00.5 1 00.5 I axinite a continuous solid solution between axinite Mg Mg and tinzenite exists only at moderately elevated tem- Fig. 4. Fe ls. Mg (a) and Mn vs. Mg (b) plots of reprcsentative peratures typical of amphibolite facies conditions. At and average tinzenite (solid square), manganaxinites (open lower temperatures a miscibility gap in the axinite circles), and ferroaxinites (solid circles). mineral series is likely to occur irrespective of bulk rock composition. intermediate between ferroaxinites and mangan- Low-temperature miscibility gaps are known to oc- axinites and tinzenites from nearby veins (Fig. 5), cur in other manganese-bearingmins14ls, such as the and it is unlikely that temperatures and pressuresof rhodochrosite-calcite isomorphous series(Goldsmith axinite-bearing vein formation have surpassedthose and Graf, 1957) and the pyralspite garn€t group of their host rocks. Furthermore, the chemical zoning (Miyashiro, 1953).In both thesegroups the break in

prehni te

umpelI yi te tinzenite istecile. \ actr nolr te Ca tourmal i ne ,'chlorite tu-Mt

end rrrenrber end nanqan nrDer - axinit f e r ro axinite

3tn. / | ,/l ,,'Manganaxinite:

| .Fer 'Fe.n91

Fig. 5. Ca-Mn-Fe (Fe + Mg) triangular diagram, showing compositional variations of analyzed axinites. PRINGLE AND KAWACHI: AXINITE

Table 3. Ranges in MnO contents for coexisting axinite and rane, tourmaline is the only boron-bearing phaserec- chlorite osnized. Mineral assemblages6palnining axinite and/or Specinen MnO axinite MnO chlorite tourmaline are portrayed on ACF-B2O, diagrams in Nunber Figure 6. We proposethat in rocks of low metamor- greenschistfacies and ou28558 0.28 - 0.85 (7) 0.0s - 0.r0(4) phic rank (chlorite zone lower), the mineral assemblagesepidote-axinite- - ou35523 r.03 - 1.5r(4) o.22 O.29(3',) chlorite and epidote-tourmaline-chlorite limit ax-

ou25335 10.45 - 12.19(s) z.+J - z.Yz\J) inite and tourmaline occurrences to rocks of restricted composition (Fig. 6a). Tourmaline can occur ou44672 core II.44 2.s6 - 3.02(3) rocks (e.g. metapelites), rim 8.78 only in relatively aluminous whereas axinite may be present in relatively cal- ou25336 11.60 - 14.41(6) 3.99 - 4.32(3) careous and mafic lithologies (e.9. metabasalts). Rocks with bulk-rock compositions within the area in Figure 6a The nunber of analyses of each phase are enclosed by axinite-epidote-tourmaline-chlorite brackets. may contain both axinite and tourmaline. The elimination of membersof the axinite group from rocks of more aluminous compositions during solid solution disappears at high temperatures. The progressive metamorphism may be represented by compositional fields of epidotes and piemontites en- equations such as: large with increasing temperatures (Miyashiro and 33Fe,.rM&.roCaoALBrSi.O'o(OH)' 1958).However, in this case,the valency states Seki, ferroaxinite of Fe and Mn (Fe'* and Mn3*) differ from those in + 19.5(Mg,Fe)o.rAl,urSi, u'O1o(OH). Mn'* of the minerals axinite, and it is the content ripidolite which increaseswith increasing temperature. + 22NaAlSi,O,+ 93SiO2+54.750.2 Host rock compositionalcontrols albite quartz Axinite-bearing veins in low-grade, regionally : 22NaFeMgrALB'SLOr'(OH)o metamorphosed rocks in southern New Zealand dravite-schorl commonly cut metabasaltsand associatedlithologies + 3TCarNrFeSirO"(OH) such as metacherts, volcanogenic metasediments,or epidote argillites. The axinites have formed in areas remote | 29CarMgrFerSi'Orr(OH),+ l9.5HrO from igneous intrusion and are clearly not of contact- actinolite metamorphic paragenesis.This relationship suggests state: that the boron necessaryfor axinite formation must Or, alternatively, without change in oxidation have been derived mainly from metabasaltic rocks or 3FerCa*ALBrSi,O3o(OH),+ 5MgrAlrSiO'(OH). associatedmetacherts or argillites under the condi- ferroaxinite amesrte tions of prehnite-pumpellyite, pumpellyite-acti- + 2NaAlSi,Os+ 5SiO' - nolite, and chlorite zone greenschist facies meta- albite quaftz morphism. + 4CarAl,Si,O,r(OH) In regionally metamorphosed rocks throughout 2NaMg,ALB3SLO"(OH). dravite zo$lte southern New Zealand tourmaline intermediate in + 2CarMgrFe,SirOo(OH),+ 5HrO composition between dravite and schorl [NaFe actinotte Mg"ALBrSLOr,(OH,F)olis widespreadas an accessory rock-forning phase and is locally abundant in Since the reactions are not univariant, due to vari- manganiferous metacherts and marbles as well as in able MgO/FeO ratios in chlorite and amphibole veins. However, in prehnite-pumpellyite, pump- phases, they may proceed over a range in temper- ellyite-actinolite, and chlorite zone greenschistfacies ature and pressure.Thus, in rocks with metamorphic rocks the association of tourmaline with axinite is rank equivalent to or higher than biotite zone green- rare. The two minerals coexist in only one sample schist facies, tourmaline can occur in lithologies with (OU25335). In rocks metamorphosedbeyond chlo- a wide range of compositions whereas axinite is re- rite zone greenschist facies in the Haast Schist ter- stricted to more Ca-rich rocktypes (Fig. 6b). PRINGLE AND KAWACHI: AXINITE b

Fig. 6. ACF-B2O3 quaternary diagrams, sssgming excess SiO, and H2O are compl€tely mobile (see text for explanation). (a) Compositional relationships between the borosilicates axinite (Ax), tourmaline (Tm), danburite @b), datolite (Dt), and dumortierite @m) and metamorphic phasestypical of the prehnite-punpellyite, pumpellyite-actinolite, and chlorite zone greenschistfacies. (Cc - : pu : calcite, Pr prehnite, pumpelMte, A6 : actinolite, Ep = epidote, Chl = chlorite). (b) Mineral assembl,agesreported in the biotite and garnet zones,greenschist facies and the amphibolite facies. Hornblende (Hb) is the stable amphibole phase.Othcr abbreviations as in 6a.

Within the high-grade part of the Haast Schist ter- Acknowledqments rane, rock compositions appropriate for axinite for- The authors thank Drs. A. Reay, M. Ozaki, I. Turnbull, and mation are rare and axinite has not been reported. W. A. Watters for kindly providing specinens for this study, and On the other hand, tourmaline is widespreadin feld- ProfessorT. Sameshimafor information regarding borosilicate lo- spathic and pelitic schists(e.g. Reed, 1958;Mason, calities in the North Island New Zealard. Discussions on initial 1962). drafts of the manuscript by ProfessorD. S. Coombs and Mr. J. Y. Bradshaw led to signifcant improvements. We have observed Thanks dso to Miss the assemblagetourmaline-cal- E. A. Gray (typing), Mr. J. M. Pillidge (polished thin sections), cite in marbles from the Torlesse,Caples, and Haast and Messrs. E. L McKenzie and A. S. Cosgrove (microprobe Schist terranes. For these rocks the interpretation of maintenanc€). mineral assemblageson ACF-BrO, diagrams cannot be applied, since SiO, is commonly not an excess References component. Andrews, P. G., D. G. Bishop, J. D. Bradshaw and G. Warren Datolite and danburite may be stable in Ca-rich (1974) Geology of the Lord Range, central Southern Alps, Ncw lithologies, provided that sufficient boron is avail- Znaland. New Zealand J. Geol. Geophys.,17,271-299. Banno, (1979) able. Vein assemblages S. and Y. Matsui Thermodynamic properties of consistingof datolite * cal- solid solutions. Earth Science Ser.,4, 63-126. Iwanani Publish- cite + q\aftz t laumontite + heulandite have been ing Company, Tokyo. described in Torlesse'greywackesfrom the central Bishop, D. G. (1972) Progressivemetamorphism from prebnite- North Island (Manion, 1974).However, datolite and pumpellyite to greenschistfacies in the Dansey Pass area, danburite have not been recorded in regionally meta- Otago, New Zealand.Geol. Soc.Arn Bull., 83,3177-3197. --, J. D. Bradshaw, C. A. Landis and I. morphosed rocks of southern M. Turnbull (1976) New Zealand, and Lithostratigraphy and structure of the Caples terrane of the their paragenetic relationships with other metamor- Humboldt Mountains, New Zealand. New Zealand J. Geol. phic phases are unclear. Dumortierite will be re- Geophys.,19,827-U8. stricted to very aluminous lithologies. It has been rec- Black, P. M. (1973)Dumortierite from Karikari psninssla;a f61 ognized in a sheared porphyritic andesite record in New Zealand. Mineral. Mag., 39,245. dyke at (1965) jVors& Karikari Carstens,H. Axinite in the Norwegian Caledonides. Peninsulia,Northland (Black, 1973), where Geol. Tidsskr., 45, 397415. it is intergrown with epidote and tourmaline. Coombs, D. S., C. A. Landis, R. J. Norris, J. M. Sintou D. J. PRINGLE AND I(AWACHI: AXINITE tt29

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