Magma-mixing in the low velocity zone : kimberlitic megacrysts from Fayette County , Pennsylvania

Two chemically distinct populations of megacrysts/inclusions are present in hypabyssalfacies kimberlite from Fayette county, S.w. Pennsylvania: A cr-rich suite and a Cr-poor suite. The cr-rich suite consists of olivine (Fo 90-93), garnet (1.64-6.14% crzo), diopside (1.89-2.34Vo Cr2O3), Cr-spinel, and immiscible sulfide-melt products. The suite possesses a limited range of mg, and major element compositions of minerals are similar to those in sheared garnet-lherzolite xenoliths. Minor element variations are consistent with crystal fractionation over a temperature interval from 1310" to 1055'C in the pressure range 48-39 kbar. The cr-poor suite is more evolved and comprises olivine (Fo 8l-85), garnet (<O.lvo Cr2O3), picroilmenite (16-37 mol% MgTiO3; 0.05-2.05% CrzOr), and diopside (<0.1% Cr2O3). This suite represents a relatively advanced stage of fractionation of a Cr-poor melt within the low velocity zone (LYZ). Mixing of these populations and their host melts has resulted in rim compositions of Fo 88-89 on both groups of olivines and reverse-zoned rims on the ilmenite megacrysts (34-51 molVo MgTiOt; | .4-3 .6% Cr2O3). The Cr-poor suite crystallized from ponded melts segregated from rising diapirs in the LVZ. Crystal fractionation in these melts led to evolved mineral compositions and ilmenite saturation. The Cr-rich suite represents crystal and immiscible sulfide-melt fractionation in a melt within an uprising diapir; the melt/crystal ratio was low and major element compositions in the melt were buffered by ambient mantle compositions (undepleted garnet lherzolite). Uprise of this melt-bearing diapir disrupted evolved, Cr-poor crystal mushes incorporating megacrysts and resulting in mixing of the host melts. Back-reaction and crystallization of the cognate xenocrysts within the hybrid melt resulted in the zoning patterns observed. peridotite xenoliths (0.2-0.4 cm); (2) they often possess compositional ranges distinct from associated mantle xenoliths; (3) calculated equilibration pressures place their origin within the depth range 100-200 km, and commonly, megacryst suites from a single intrusion display a wide range of equilibration temperatures, typically 200-300"C (e.9., Pasteris et al.,1979); and (4) temperature-related mineralogical and textural features frequently are present (e.g., Boyd and Nixon, 1975). These textural and mineralogic characteristics suggest that an origin by disaggregation of mantle peridotite is unlikely and it is a widely held view among kimberlite petrologists, that these features are consistent with high-pressure crystallization from fractionating melts within the low velocity zone (LYZ). A principal point of contention is the relationship of these megacrysts to their host kimberlite. Are they xenocrysts (e.g., Boyd and Nixon, 1973; Pasteris et al., 1979; Pasteris, 1980) or phenocrysts in a kimberlitic liquid (e.g., Dawson and Stevens, 1975; EggJer et al.,1979; Garrison and Taylor, 1980; Gurney er al.,1979; Mitchell, 1973; Mitchell and Clarke, 1970? In HUNTER AND TAYLOR: MEGACRYSTS FROM PENNSYLVANIA l 7 addition, the origin and significance of rim-zonation in olivine (e,9., Boyd and Clement, 1977;. Emeleus and Andrews, 1975) and ilmenite megacrysts (e.g., Agee et al.,1982; Boctor and Boyd, 1980a; Haggerly et al.,1979; Pasteris, 1980) have also provoked debate. Fresh, hypabyssal-facies kimberlite from S. W. Pennsylvannia contains a well-preserved megacryst and inclusion suite typical of kimberlites in general. Two chemically distinct populations of megacrysts and inclusions are present, a Cr-rich suite and a Cr-poor suite; these are analogous to those described from the ColoradeWyoming kimberlites by Eggler et al. (1979). The Cr-rich suite consists of olivine, garnet, orthopyroxene, clinopyroxene, Cr-spinel, and sulfides; this group shows minor element compositions that vary systematically with equilibration temperatures and pressures, although mg shows a restricted range. The Cr-poor suite consists of olivine, garnet, ilmenite, and clinopyroxene; these phases possess restricted compositional ranges but are significantly more evolved than the Cr-rich suite. Olivines in both groups possess identical rim compositions, and the ilmenite megacrysts of the Cr-poor suite display reverse zonation. These features are interpreted within the context of a magma-mixing model. This model is incorporated with aspects of models presented by other authors into a reasonably coherent scenario for evolution of kimberlitic melts within the LVZ. Petrography and mineral chemistry Megacrysts and xenoliths are set in a matrix largely composed of phlogopite, ilmenite, Cr-spinel-titanomagnetiterro perovskite, and carbonate with subordinate serpentine, apatite, rutile, and sulfides. The petrology of these groundmass phases, together with the location and general characteristics of the Fayette County kimberlite are described in a companion paper (Hunter et al.,1983). Groundmass grains are less than 2 mm in size. Megacrysts are rounded to subangular, and typically, dimensions are in the range 0.5-1.5 cm, but grains are present up to 3 cm. The term megacryst is used herein to describe optically continuous, single-phase grains or partial mosaics. Inclusions are single grains (except in the case ofthe sulfide blebs) within megacrysts, typically 0.5-1.0 mm in size, and are rounded to subhedral in form. One cm is usually considered to be the lower size limit of megacrysts sensa sticto (e.g., Harte, 1978). Commonly, in kimberlites where there is a distinct bimodal distribution of grain sizes, the lower limit of the large grain-size population is around one cm. However, in the discrete (i.e., non-xenolithic) olivine population, as a whole, there is a seriate size distribution (<2 mm to 3 cm), skewed towards the smaller size range. The cores of all the olivine grains less than 2 mm in size show chemical identity with the Cr-rich, olivine-megacryst population. Commonly, the smallest olivine grains and olivine inclusions fall within the size range exhibited by the other megacryst inclusions, and clearly, there is an unambiguous cogenetic relationship between megacrysts and inclusions. Considerations of grain size distribution and chemistry of the olivines suggests that it would be misleading to consider the smaller olivine grains in isolation from the megacrysts, and usage of the term olivine herein includes the entire discrete olivine population. Olivines with subordinate garnet comprise the phases in the Cr-rich suite; in order of decreasing abundance, inclusions consist of (Table l): orthopyroxene, sulfide, olivine, garnet, clinopyroxene, and Cr-spinel. Within the Cr-poor suite, three phases exist as megacrysts. Relative modal abundances of these are olivine > ilmenite > garnet; ilmenite inclusions are more abundant than clinopyroxene. As a group, the Cr-rich suite is relatively more abundant than the Cr-poor suite, by a factor of 2-3. Phlogopite megacrysts are rare and may or may not belong to one of the two groups. Ultramafic xenoliths are not common; their characteristics are described briefly, but they are only discussed within the context of the megacryst evolution. Mineral analyses were performed using an automated MAC 4005 electron microprobe, using data reduction procedures of Bence and Albee (1968) and the data of Albee and Ray (1970). Whenever possible, 5-10 analyses were performed on each grain, and means and standard deviations computed. Core regions of grains were homogeneous, but zonation was present in the outer 100-200 pm of olivine and ilmenite megacrysts. Representative analyses of silicates and oxides are presented in Tables 2 through 6; ranges of chemical parameters of minerals in the Cr-rich and Cr-poor suites and in garnet-lherzolite xenoliths are summarized in Table 7.