Petrology of the hypabyssal kimberlite of the Kroonstad group II kimberlite (orangeite) cluster, South Africa: Evolution of the magma within the cluster

Geology Department, Rhodes University, P.O. Box 94, Grahamstown, South Africa
Lithos (Impact Factor: 4.48). 07/2011; 125(1-2):795-808. DOI: 10.1016/j.lithos.2011.05.001

ABSTRACT We report here a detailed petrographic study accompanied by the first whole-rock major and trace element geochemical analyses as well as the first Sr and Nd isotope data for the Kroonstad Group II Kimberlite Cluster. Hypabyssal kimberlite is described from the Lace, Voorspoed and Besterskraal North kimberlite pipes. The Lace kimberlite is a monticellite phlogopite kimberlite with mineralogy and geochemistry more similar to typical Group I kimberlites. The Voorspoed kimberlite is an aegirine and sanidine bearing kimberlite and along with the geochemistry shows a more evolved composition relative to the Lace kimberlite. The Besterskraal North kimberlite is a highly evolved type Group II kimberlite containing abundant K-richterite, sanidine, aegirine and leucite. The magma evolutionary trend is linked to a variation in olivine macrocryst abundance. The Lace unevolved magma contains abundant olivine macrocrysts and has not been affected by significant fractionation whereas the Voorspoed and Besterskraal North kimberlite contain far fewer olivine macrocrysts, which indicates that significant fractionation has affected these magmas en route to the surface. We evaluate the potential for fractionation to result in differentiation of a common parent magma or magma generated from a common source en route to the surface. This process is complicated by the fact that olivine macrocrysts are essentially xenocrysts within the magma and the loss of these xenocrysts will not affect the primary magma composition. Furthermore we present two alternate hypotheses for the evolution of the magmas; a) contamination en route to the surface and b) varying degrees of partial melting at the source. The evolutionary trend is also linked with diamond characteristics. In particular diamond parcels at the Voorspoed pipe between 1906 and 1911 were deficient in large stones, which indicate that fractionation has likely also removed the large stones from the Voorspoed magma along with the olivine macrocrysts.Research highlights► Kimberlite magma evolution within a single cluster. ► Evolution linked to olivine macrocryst/phenocryst abundance a ratio. ► Evolution linked to possible early contamination by crustal material. ► Identification of pseudomorphed monticellite and melilite in a Group II kimberlite.

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    ABSTRACT: The Voorspoed Group II kimberlite pipe is atypical in terms of South African kimberlite pipes. Located in the central region of the Kaapvaal Craton (South Africa), the Voorspoed pipe is one of six pipes comprising the Kroonstad Kimberlite Cluster. Reconstruction of the palaeo-stratigraphy at the time of emplacement, of the Kroonstad kimberlites, indicates that significant post-emplacement erosion has occurred and the pipes are currently exposed at ~ 1600 m depth from the original land surface. The volcaniclastic kimberlite (VK) infilling the pipe is distinctly different from typical tuffisitic kimberlite infilling other South African type kimberlite pipes. Three textural volcaniclastic kimberlite varieties are observed: massive volcaniclastic kimberlite (MVK), bedded volcaniclastic kimberlite (BVK) and fine-grained volcaniclastic kimberlite (fgVK) layers. The BVK and fgVK are volumetrically insignificant and the bulk of the pipe is infilled with MVK. The MVK can be further sub-divided into seven varieties. The two dominant varieties are described in detail, which include: main MVK (mMVK) and the juvenile-rich MVK (jrMVK). The MVKs are typically massive, clastic, poorly sorted and supported by an interclast material consisting of juvenile and xenocrystic crystals in a mud-rich matrix. Magmaclasts within the MVKs are spherical, crystalline with a typical coherent hypabyssal texture and different mineralogical varieties are typically juxtaposed. BVK units are composed of several normally graded beds of volcaniclastic material. fgVK layers are well sorted relative to the MVKs and all constituents are typically < 2 mm in size. The BVK and fgVK can be deposited only within an open vent in order to account for the sorting of the components. A distinct basalt and sandstone rich unit (bsBreccia), which is essentially devoid of kimberlite material, is also observed. Three volcaniclastic zones are identified based on consistent vertical variation in the internal stratigraphy of the infill: outer east/west zone with a sequence bsBreccia–BVK–mMVK/softMVK; a central zone with a sequence mMVK–BVK–jrMVK–mMVK and an outer north zone with a uniform mMVK sequence.The evidence suggests that the volcaniclastic infill has formed through complex processes involving re-working of extra-crater tephra back into the vent and coeval pyroclastic deposition. It is also likely that there is significant mixing of the resedimented and pyroclastic deposits. Distinction between deposits formed through re-working of material through repeated explosive eruptions within the vent and that formed through varying degrees of re-working extra-crater tephra from the surface (with possible coeval pyroclastic eruption) is probably impossible. For this reason we interpret the mMVK as being deposited by dominantly resedimentation of tephra whereas the jrMVK has been deposited dominantly by pyroclastic processes. The presence of BVK and fgVK deposited deep within the pipe through slumping of the crater margins at the surface suggests that a crater ~ 2000 m in depth was periodically open.The identification of the complex, layered, variable internal structure of the volcaniclastic infill at the Voorspoed has significant implications for diamond ore evaluation in South Africa. Typical South Africa kimberlite pipes are infilled with tuffisitic kimberlite, which are characterised by their homogenous nature over large vertical intervals (> 1000 m). This is not the case at the Voorspoed pipe where MVK layers are variable in thickness (< 10 m up to 230 m).
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    ABSTRACT: We report carbonate-and silicate-rich globules and andradite from the Wajilitage kimberlitic rocks in the northwestern Tarim large igneous province, NW China. The carbonate-rich globules vary in size from 1 to 3 mm, and most have ellipsoidal or round shape, and are composed of nearly pure calcite. The silicate-rich globules are elliptical to round in shape and are typically larger than the carbonate-rich globules ranging from 2 to several centimeters in diameter. They are characterized by clear reaction rims and con-tain several silicate minerals such as garnet, diopside and phlogopite. The silicate-rich globules, reported here for the first time, are suggested to be related to the origin of andradite within the kimberlitic rocks. Our results show that calcite in the carbonate-rich globules has a high X Ca (>0.97) and is characterized by extremely high concentrations of the total rare earth elements (up to 1500 ppm), enrichment in Sr (8521–10,645 ppm) and LREE, and remarkable depletion in Nd, Ta, Zr, Hf and Ti. The calcite in the sili-cate-rich globules is geochemically similar to those in the carbonate-rich globules except the lower trace element contents. Garnet is dominantly andradite (And 59.56–92.32 Grs 5.67–36.03 Pyr 0.36–4.61 Spe 0–0.33) and is enriched in light rare earth elements (LREEs) and relatively depleted in Rb, Ba, Th, Pb, Sr, Zr and Hf. Phlogopite in the silicate-rich globules has a high Mg # ranging from 0.93 to 0.97. The composition of the diopside is Wo 45.82–51.39 En 39.81–49.09 Fs 0.88–0.95 with a high Mg # ranging from 0.88 to 0.95. Diopside in the silicate-rich globules has low total rare earth element (REE) contents (14–31 ppm) and shows middle REE-(Eu to Gd), slight light REE-and heavy REE-enrichment with elevated Zr, Hf and Sr contents and a negative Nb anomaly in the normalized diagram. The matrix of the kimberlitic rocks are silica undersaturated (27.92–29.31 wt.% SiO 2) with low Al 2 O 3 (4.51–5.15 wt.%) and high CaO (17.29–17.77 wt.%) contents. The samples are characterized by incompatible element enrichment with high (La/Yb) N values (41–58) and remarkable negative anomalies in HFSEs (e.g. Ta, Zr, Hf). Our new data suggest that the carbonate-rich globule most likely crystallized at high-temperature and does not represent immiscible liquids, whereas the silicate-rich globules are related to carbonate-rich deuteric hydrothermal fluids during the later-stage of melt evolution. The fluids reacted with the surrounding silicate melts resulting in the formation of skarn minerals such as phlogopite, diopside and andradite. The presence of the carbonate-bearing globules indicates that the Wajilitage kimberlitic rocks are carbonate-rich and most likely derived from an enriched mantle with abundant carbonate. We correlate the carbonated mantle to metasomatism by the migration of deep-seated fluids (carbonate-rich) in response to the impingement of the early Permian mantle plume.