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Crystallization History and Textures of the Rearing Pond Gabbro,Northwestern Wisconsin

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Crystallization History and Textures of the Rearing Pond Gabbro,Northwestern Wisconsin American Mineralogist, Volume 64, pages 844-855, 1979 Crystallization history and textures of the Rearing Pond gabbro,northwestern Wisconsin JauEsF. Or-rrrsreo Department of Earth Sciences State University College of Arts and Science Plausburgh, New York 12901 Abstract The Rearing Pond intrusion is a small basicbody that displaysa rangeof compositionand textures illustrating its cooling history. Modal and mineralogical data indicate that it is a singlediflerentiated unit or a cyclic unit in a larger poorly exposedintrusion. Fractionation has resultedin a successionthat varies from olivine cumulatesthrough oli- vine-plagioclase-clinopyroxenecumulates to plagioclaseclinopyroxene cumulates. Composi- tional variation of phasesis small, with iron enrichment about l0 mole percentin the mafic phasesand with plagioclasevarying from An 80 to An 60. Somezoning of plagioclaseis ap- parent,becoming stronger in the later units. Primary clinopyroxene flrst appears as large oikocrysts in the olivine-plagioclase cumu- lates,and later changesto a granular habit in the olivine-free rocks. This is interpretedas a changein the ratio ofgrowth to nucleation ratesofthe pyroxenecorresponding to the cessa- tion of olivine crystallizationas the magma compositionleaves the olivine field. This occurs at the quaternary invariant point, Fo-An-En-Di-Liquid in the systemAn,Di,Fo,SiOz. The habit changeoccurs at this point as a result ofthe changein degreeofsupersaturation as the liquid fractionatespast the invariant point as olivine is consumed.This is either a kinetic ef- fect or a compositionalone due to the diflerencesin competition for componentson opposite sidesofthe invariant point. Evidencesuggests that both nucleationand growth acceleratedin the latestrocks, as grain sizeis diminishedand zoning becomesmore pronounced. Introduction with fractionation of a basaltic magma. In this paper and related textural The textures of basic igneous rocks have received the changes in cumulus minerals compared with the considerable attention through the years. Some no- changes have been charted and by the petro- table examples have dealt wittr the ophitic texture mag|a composition' as determined to explain (Krokstrom, 1932; Walker, 1957; Wagir, 196l; op- g-raRhic analysis of the rocks' in an effort in that have oc- penheim, 1964). Others (Wager et al., 1960; He;s, the textural types and changes type is a small Keweenawan ieOO; W"g"r, 1963) studied features of igneous sedi- curred' The example used intrusion which displays a textural change mentation and the processes responsibie for them, gabbro granular that appears related to and Jackson (1967) developed a classification based from ophitic to composition resulting from fractionation on these processes.More recently a number of change in. magma' Mineral compositions deter- imental studies based on classical theory of crystal"*f"r- of. a basaltic end-member compositions' nucleation and growth have provided new insigtrt min-ed are approaching model provided by Osborn and Tait is an into the relationship between irystal habit, igneJus so that the excellent starting point for development of an hy- textufes and cooling history (Kirtpatrick , D7Z; Lof- to magma composition' It is gren, 1974;Swanson, 1977;Fenn, 1977). pothesis relating texture that a starting magma composition could It appears that textural relationships observed in unfortunate not be determined for the intrusion' these iock types are not only a function of rate of cooling but may also be controlled by the composi- Geologic setting tion ofthe magma, which in turn is often the result of the degree of fractionation. The quaternary system The Rearing Pond intrustion is one of several that anorthite-forsterite-diopside-silica (Osborn and lie along the southern boundary of the Keweenawan, Tait, 1952) illustrates the phase changes that occur Lake Superior syncline. The most cornmon intrusive w3 -w4x /7 9/ 0708-08,14$02.oo OLMSTED: REARING POND GABBRO 845 EXPLANATION REARINGPOND INTRUSION ADJACENTUNITS euarrzGabbro [I,.T-.TI Granite ffi Troctorite& Gabbro E"l l- Peridotite #lHI"- F:r\sil Wisconsin d Basart = IntrusiveContact (inferred) -- -_ - GradationalContact lgneousLayering Index Map mi Scale 10 1.0 Fig. l. Geological map of Rearing Pond intrusion. Rock units south of the intrusion are part of the Mineral Lake intrusion. rocks are gabbro, anorthositic and troctottic gabbro, called the Mineral Lake intrusion (Olmsted, 1968), and micrographic granites. Other related intrusive which is about 5000 m thick and 18 kn long. rocks, such as peridotite and occasional intermediate The Rearing Pond intrusion occupies a con- rocks,are found, but they are rare. The detailsofthe spicuous embayment in the top of the Mineral Lake structure and stratigraphy of the Keweenawan and intrusion. Intrusive relationships between the two older Precambrian units of the region have been de- units are not clear, as the contact is poody exposed. scribed by Aldrich (1929) and Leighton (1954),and Both units, however, intrude the Lower Keweenawan severalmore recent reviews have been of value in un- volcanics, which in some caseshave been so engulfed derstanding both local and regional problems (Halls, that they are now little more than thin screensbe- 1966;White, 1966;Weiblen and Morey, 1975).These tween intrusives. Figure I shows the Rearing Pond and other studies (Olnsted, 1968; Hubbard, 1968, intrusion as a lens-shapedbody tapering to the east 1975;Books et al., 1966) suggestthat the intrusive between other igneous units and to the west in the units are tabular bodies emplaced along surfaces volcanics. The orientation of ig.eous laminations such as faults or unconformities and are of Middle and layering within the intrusion is shown in Figure Keweenawanage. Along its south limb the Keweena- 2, and suggeststhat the form of the body is that of an wan units of the Lake Superior syncline dip north- elongate funnel tilted to the northwest. westerlywith moderateto steepangles, exposing the Figure 2 shows that when the regional dip is re- units in cross section. The entire volcanic section. in- moved the dips of the internal structures all converge cluding the intrusives in this area, is on the order of to a point beneath the center of the intrusion. The 8000 to 10000m thick. In the Mellen area the intru- distribution of rock types also seems to indicate a sions have been emplaced into the Lower Keweena- tilted funnel or cone-like structure. The lowermost wan lavas and have nearly engulfed them. The major unit is a feldspathic peridotite which forms a layer or intrusion west of Mellen is a large anorthositic unit envelope about the centrally-situated main mass of 846 OLMSTED: REARING POND GABBRO are shown in Table I and Figure 3. The earliest unit in a fractionation sequenceis the peridotite (an oli- vine cumulate) that forms the envelope around the central unit. Next are the troctolitic rocks (olivine- plagioclase cumulate, with and without pyroxene oikocrysts), which form much of the central part of the intrusion and account for about eighty percent of its exposed area. The uppermost unit is quartz gab- bro (plagioclase-clinopyroxene cumulate) found in the northwestern corner of the intrusion. Actual con- Table l' Modal analyses pERrDorrrE w-204 O11v1ne* 72.4 PlaSioc lase 24.3 23.2 Diotite o.7 0rEhopyroxene 0.2 7.7 Cllnopyroxene 2.2 4.7 Opsque I.I 4,2 Nwber of counts 3500 3245 Leuco- l,laflc- Spotted TloctoliEe Troctolite Gabbro Noricic w-125L W-I63 !l-39 w-46 Fig. 2. Structural intcrpretation of igneous layering data and Ollvlne* 23,4 39.7 13,8 3. 1 rock type distribution. Upper: Measured orientation of layering Plsgioclase 72.4 52,a 54.6 60.1 shown at tails of arrows. Renoval of regional dip by.1661ie1 Biocite - ht about horizontal axis, which is shown as dashed line. results in the OrEhopyroxeu€ orientations shown at heads of arrows. The 44o average dip .2.1 2.2 5.4 20.9 located in the southwest corner of the intrusion, the 20o dip at the Clinopyroxene 0.5 5.1 25.6 9.4 northwestern end, and the 40" dip in the east c€trtral area are APstite - 0.1 shown in Fig. l. The result is the new strikes all roughly parallel Secondary the perimeter of the body with dips toward the center, as shown in Chlorite or - 0.6 - 1.4 trornblende th€ cetrter diagram. This 60" rotation of all values is justifed by a 60o average regional dip of the Keweenawan sedinentary and l,lagnerite 0.6 0.6 0.6 3.1 volcanic units. The lower diagram is an interpreted north-south Nudber of couts 1400 1555 9800 1600 crosssection, rcstored to the original orientation. w-6 w-I26 w-19 w-I64 w-166 troctolitic rocks. The uppennost unit is a quartz gab- Plagloclase 55.6 48.7 61.7 52.7 55.0 bro which is clearly the product of fractionation of a Cllnopyroxene 13.2 30.6 14.l 25.5 34.O more basic magma. In general, layering is not wide, orthopyroxene L5.7 3.9 12.9 7.6 8.9 spread, but in the higher parts of the intrusion some olivine* 0.5 15.8 2.1 rhythmic layering, both graded and unimodal, has Biottte 0.I - 0.8 3.2 been observed.The implication of this lack of small- lfagn€tice 0.3 0.9 0.5 4.3 0.8 scale layering in the lower units is that convection Apatite Tr Tr Tr o.4 Tr was either very slow or not periodic until the later Aophibole 74.6 - 7.9 1.0 1U stageof the igneous period. cooling quartz 3.0 Petrographic descriptions Alkeli Feldspar Zircon Tr The Rearing Pond intrusion has been divided into Nuniber of counts 1400 1800 1548 L6t2 three units that are petrographically distinct and *Serpentlne was always co@ted as olivine. In sme exanples olivine was readily recognized in the field. Mineral compositions as much as 90% serpencinized, OLMSTED: REARING POND GABBRO 847 tacts between the units are rare, and field studies have confirmed that most changesfrom one type to anothertake placeover a narrow transition zone.
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