Investigation of polyphase and fluid inclusions in clinopyroxene from diamond-grade calc-silicate rocks of Kokchetav Massif (Northern Kazakhstan)

Mikhno A.O.

V.S. Sobolev Institute of Geology and Mineralogy sb ras, Novosibirsk, Russia

mao14@list.ru

By far the discovery of melt inclusions associated with diamond in carbonate rocks of Kokchetav massif renewed interest of scientists to the nature of melt in UHPM rocks [De Corte, 1998; Korsakov & Hermann, 2006]. These findings provided the strong evidence that presence of melts promotes diamond growth in deeply subducted crust. Previous study of fluid and polyphase inclusions was mainly focused on diamond-bearing rocks.

In this paper we present preliminary results on phase and chemical composition of polyphase inclusions from clinopyroxenes of diamond-free calc-silicate rocks from Kokchetav massif. Lens and interlayers of garnet-clinopyroxene (calc-silicate) rocks occur among biotite gneisses and schists [Shatsky et al., 1995]. Calc-silicate rocks are composed by garnet, clinopyroxene, calcite, quartz, plagioclase, K-feldspar. Titanite, zircon and pyrrotine occur as accessory minerals. Samples contain a compositional layering consisting of different amounts of coarse- to medium- grained clinopyroxene (30-80 vol. %) and garnet (20-50 vol. %) in calcite matrix. Garnets are characterized by following composition: Alm₁₀Sps₃Pyr₁Grs₇₇Andr₉. Clinopyroxenes shows high diopside content. Cores of large porphyroblasts of clinopyroxene (up to 4 mm in size) contain abundant lamellas of KAlSi3O8 and coexisting polyphase and fluid inclusions, whereas the rims are free of lamellas and inclusions. Isolated inclusions showing the shape of negative crystal are primary inclusions. Raman-imaging combining with scanning electron microscopy allow to identify following phases in the representative inclusions: KAlSi₃O₈ (kokchetavite), calcite, muscovite/phengite, H₂O-liquid.

According to obtained data we assume that originally polyphase inclusions were trapped as a melt. During clinopyroxene growth there were two immiscible liquids: fluid and melt. It confirms by findings of coexisted fluid and polyphase inclusions within the same growth zone of clinopyroxene porphyroblast. Crystallization daughter phase calcite and mica from these melt inclusions during retrograde stage liberates liquid H₂O. The differences between Raman spectra for clinopyroxene in direct proximity to the polyphase inclusion and host-clinopyroxene indicate that there are differences in chemical composition within this thin zone. Formation of this zone can be related either to interaction of melt/fluid with host-clinopyroxene or to precipitation of newly formed clinopyroxene on the wall of melt/fluid inclusions.

This study was supported by the RFBR grants (10-05-00616-а).

Crustal and mantle-derived peridotites from hp-uhp collision belts: mineral composition and geochemical differences

Selyatitskii A.Yu.

V.S. Sobolev Institute of Geology and Mineralogy sb ras, Novosibirsk, Russia

dyadyasasha@ngs.ru

Basic and ultrabasic rocks in the high- and ultrahigh-pressure collision belts can provide important petrological information. Among such rocks in Phanerozoic orogenic zones are distinguished the mantle-derived and “crustal” peridotites and pyroxenites. Mantle-derived peridotites (Mg-Cr type) are represent restite mantle and were emplaced as tectonically transferred solid fragments of mantle material in the deeply subsided lithosphere and had preserved all geochemical features of such rocks, while the crustal ones (Fe-Ti type) have had ultrabasic-basic pre-collision low pressures protholiths (for example shallow complexes of dikes and sills) which were could be strong metosomatized and then, during subduction process, metamorphosed. Both rock types were later exhumed at the Earth’s surface.

Geochemical investigation has revealed that mantle-derived peridotites are characterized by the high contents of MgO (35-46 wt. %), Cr (1750-12770 ppm) and Ni (900-2500 ppm) and low contents of FeO (5-10 wt. %), TiO₂ (0.01-0.3 wt. %), Zr (0.002-1.2 ppm), Nb (0.001-0.3 ppm), total of rare-earth elements – REE (0.06-5.2 ppm). “Crustal” peridotites are characterized by the high contents of FeO (12-25 wt. %), TiO₂ (0.64-2.6 wt. %), Zr (33-179 ppm), Nb (3.4-13.8 ppm), Sm (0.7-4 ppm), total REE (11.5-48 ppm) and comparatively low contents of MgO (15-26 wt.%), Cr (79-244 ppm) and Ni (450-730 ppm) [1].

	FeO	MnO	MgO	TiO2	Cr2O3	NiO	f
Ol	20 (13-38)	0,22 (0,02-0,85)	41 (26-46)	0,02 (0-0,12)	0,01 (0-0,15)	0,02 (0-0,17)	0,19 (0,14-0,33)
	9 (4-13)	0,12 (0,01-0,24)	50 (46-55)	0,01 (0-0,1)	0,02 (0-0,43)	0,38 (0-0,58)	0,09 (0,04-0,14)
Opx	12 (9-18)	0,2 (0,03-0,43)	30 (20-33)	0,07 (0-0,17)	0,01 (0-0,06)	0,01 (0-0,05)	0,18 (0,13-0,32)
	6 (3-9)	0,14 (0,01-0,28)	34 (30-38)	0,09 (0-0,36)	0,31 (0-0,91)	0,07 (0-0,17)	0,09 (0,054-0,14)
Cpx	4 (2-7)	0,05 (0,01-0,23)	16 (13-18)	0,05 (0,01-0,11)	0,17 (0-0,9)	0 (0-0,03)	0,12 (0,07-0,02)
	2 (1-4)	0,08 (0-0,33)	16 (14-19)	0,21 (0-0,97)	1,15 (0,19-3)	0,05 (0-0,53)	0,07 (0,03-0,12)
Grt	16 (13-24)	0,36 (0,15-0,9)	15 (10-18)		0,12 (0-1,33)	0,01 (0-0,04)	0,37 (0,29-0,57)
	10 (6-14)	0,47 (0,01-0,87)	20 (16-23)		2,1 (0,34-4,8)	0,02 (0-0,09)	0,21 (0,12-0,33)
Spl	19 (16-29)	0,12 (0,05-0,25)	16 (11-18)	0,01 (0-0,03)	0,07 (0-0,25)	0,01 (0-0,04)	0,41 (0,33-0,61)
	18 (10-42)	0,18 (0-0,45)	15 (4-21)	0,16 (0-0,73)	26 (4-55)	0,17 (0-0,78)	0,42 (0,21-0,86)

Table. Average, minimum and maximum contents of elements in minerals from mantle and “crustal” peridotites.

Ol – olivine, Opx – othopyroxene, Cpx – clinopyroxene, Grt – garnet, Spl – spinel, f = Fe/(Fe+Mg). A numerator is “crustal” peridotites, denominator is mantle peridotites. Number before brackets is an average value, numbers in brackets are min and max values. Total number of analysis are: 184 – for “crustal”, 466 – for mantle.

Furthermore the genetic type of “crustal” peridotites represented by garnet and spinel ultrabasic rocks of the Kokchetav massif (Northern Kazakhstan), Dabie-Sulu terrain (Eastern China), and the Western Gneiss Region (Norway) is characterized by a number of mineral peculiarities, which distinguish it from mantle (Alpine-type) peridotites [2, 3]. First of all, these are the high iron number (f = Fe / (Fe + Mg)) and low concentrations of Cr₂O₃, NiO, CoO in the studied olivines, orthopyroxenes, garnets, clinopyroxenes and spinels (see table). These peculiarities radically distinguish them from minerals of ultrabasic rocks of the typical mantle origin. The different concentrations of FeO, MgO, Cr₂O₃, NiO, CoO in minerals from the mantle and “crustal” sets provide evidence for the differences in the bulk composition of peridotites of different genetic types. The higher concentrations of titanium in mantle clinopyroxenes, spinels, and orthopyroxenes in comparison with the “crustal” ones are most likely explained by the fact that these minerals in mantle peridotites may be practically the only concentrators of the TiO₂ admixture, whereas the high bulk concentration of titanium and iron in “crustal” peridotites results in appearance of significant portions (n%) of an individual Ti bearing phase (ilmenite). This also explains the similar concentrations of manganese in pyroxenes, olivines, spinels, and possibly garnets. In addition to the bulk chemical compositions of rocks, the revealed differences in the chemical compositions of minerals from mantle and “crustal” peridotites may be applied for diagnostics of ultrabasic rocks in Precambrian cratons and collisional zones of high/ultrahigh pressures with respect to the origin of rocks and establishment of the nature of their protoliths.

This work supported by Russian Foundation of basic research.

References:

1. Reverdatto V.V., Selyatitskii A.Yu., Carswell D.A. (2008) The geochemical distinctions between “crustal” and mantle-derived peridotites/pyroxenites in the high/ultrahigh pressure metamorphic complexes // Rus. Geol. Geophiz. V. 49. P. 73-90.

2. Selyatitskii A.Yu., Reverdatto V.V. (2011) Comparison of the compositions of olivines and orthopyroxenes from mantle and “crustal” peridotites of collisional high pressure / ultrahigh pressure zones. // Dokl. Earth Sci. V. 438. Part 1. P. 705-710.

3. Selyatitskii A.Yu., Reverdatto V.V. (2011) Comparison of the compositions of clinopyroxenes, garnets, and spinels from mantle and “crustal” peridotites of collisional high pressure / ultrahigh pressure zones. // Dokl. Earth Sci. V. 441. Part 2. P. 1695-1702.