Alan R. Woolley

Nomenclature of amphiboles : additions and revisions to the International Mineralogical Association's amphibole nomenclature

The introduction of a fifth amphibole group, the Na-Ca-Mg-Fe-Mn-Li group, defined by 0.50 < B(Mg,Fe2+,Mn2+,Li) < 1.50 and 0.50 ≤ B(Ca,Na) ≤ 1.50 a.f.p.u. (atoms per formula unit), with members whittakerite and ottoliniite, has been required by recent discoveries of B(LiNa) amphiboles. This, and other new discoveries, such as sodicpedrizite (which, here, is changed slightly, but significantly, from the original idealized formula), necessitate amendments to the IMA 1997 definitions of the Mg-Fe-Mn-Li, calcic, sodic-calcic and sodic groups. The discovery of obertiite and the finding of an incompatibility in the IMA 1997 subdivision of the sodic group, requires further amendments within the sodic group. All these changes, which have IMA approval, are summarized.

PYROCHLORE FROM WEATHERED CARBONATITE AT LUESHE, ZAIRE

Abstract A detailed study of weathered pyrochlore in the laterite above carbonatite at Lueshe, NE Zaire, has been made in order to determine its chemical and textural variations. Pyrochlore in fresh carbonatite at Lueshe is close to an ideal formula of (Ca,Na)2Nb2O6(OH,F) (where a general formula is A2−xB2O6(OH,F)1−y·zH2O). The first and principal change on weathering occurs at the base of the profile and involves the leaching and partial exchange of A cations together with hydration. This change appears common to weathered pyrochlore worldwide. As a result weathered pyrochlore at Lueshe has a large apparent A cation deficiency with A totals between 0.25 and 0.59. The B cations remain stable. Abundant kalipyrochlore is unique to Lueshe and is thought to be related to the abundance of potassium feldspar in the fresh carbonatite, showing that the actual composition of weathered pyrochlore is a characteristic of a particular deposit. Weathered profiles at I,ueshe are not simple trends from the least to most leached compositions. Further factors including variation in whole rock mineralogy and chemistry, and cation exchange and uptake are responsible for local concentrations of strontio-, bario- and calcium-rich, sodium-poor pyrochlore in the ore body, as well as rims of ceriopyrochlore on kalipyrochlore. The most important textural relationship in the Lueshe pyrochlore is the intimate intergrowth with crandallite in the most weathered parts of the laterite. Although pyrochlore persists throughout the weathering profile, niobium-beating goethite is thought to represent the final product of pyrochlore breakdown.

Petrography and mineral chemistry of mantle xenoliths in a carbonate-rich melilititic tuff from Mt. Vulture volcano, southern Italy

Abstract We present petrographic and mineralogical data for 21 mantle xenoliths (12 lherzolites, 8 wehrlites and 1 composite) selected from a suite of more than 70 samples collected from the Monticchio Formation, Mt. Vulture volcano, southern Italy. The xenoliths are rounded, coarse- to porphyroclastic-textured, and very fresh, with the following equilibrated mineral assemblages; olivine (Fo90−92), orthopyroxene (~En89, Wo2.0), clinopyroxene (Mg90−92, 3−6% Al2O3, 1−1.5% Cr2O3), and chrome-spinel (14−20% MgO, ~30−40% Cr2O3). Many xenoliths contain partial melt glasses and accessory sulphide (pentlandite). Some contain primary mica (phlogopite with ~4% FeO, 1.8% Cr2O3, 1.4−2.8% TiO2) with slightly zoned rims (Fe-, Ti-, Al-enriched). One contains relics of garnet (pyrope; Mg84). Secondary veins in several xenoliths contain carbonate with significant Sr levels (~0.5−1.0% SrO), occasional apatite and scarce melanite, all typical of carbonatites and presumably related to the host magma (melilitite/carbonatite). Although amphibole is a common megacryst in the same volcanic units, no primary amphibole was found in the xenoliths themselves. Calculated pressures and temperatures using a range of geothermometers/barometers give values of 14−22 kbar and 1050–1150°C. In particular, the En-Sp and Di-Sp thermo/barometers (Mercier, 1980) show a good positive correlation between P and T. The Monticchio xenoliths lie on the high-T side of an ‘oceanic’ geotherm. The xenolith geotherm is hotter than general heat flow values in this region at the current day (50 mWm-2) but it compares well with the high-pressure end of a typical alkaline continental rift.

Geochronology and cooling history of the northern part of the Chilwa Alkaline Province, Malawi

Abstract The Cretaceous Chilwa Alkaline Province of southern Malawi has an exceptional variety of lithologies ranging from carbonatite to granite. The largest plutons consist of syenite and peralkaline granite, with somewhat smaller intrusions consisting of syenite, nepheline syenite and sodalite syenite. Carbonatite and minor nephelinite occur on Chilwa Island and metamorphosed basanitic/nephelinitic volcanics are preserved in down-faulted blocks. KAr amphibole and titanite fission-track ages were determined for all of the major plutons and from these ages emplacement times were estimated for the various plutons. Apatite fission-track ages and confined track-length measurements in apatite were determined in order to develop a thermal model for the Chilwa Province. KAr amphibole and titanite fission-track ages indicate that igneous activity started at ca 133 Ma with the extrusion of nephelinitic lavas and the emplacement of nepheline and sodalite syenites. An existing KAr date for the carbonatite at Chilwa Island gives a similar age. Nepheline syenites and syenites were intruded at ca 126 Ma followed by the emplacement of large syenite-peralkaline granite plutons at ca 113 Ma. The geochronology suggests that there was a temporal variation in the chemistry of the magmas forming the Chilwa plutons. Apatite fission-track ages and track-length measurements indicate that the intrusions were emplaced at high level (shallow burial depths) and cooled relatively rapidly to near ambient temperatures. The emplacement of the large syenite-granite pluton (Zomba-Malosa) caused local doming, which led to a rapid exhumation rate for Zomba-Malosa. Plutons distal to Zomba-Malosa show a slower cooling rate, suggesting that they were on the flanks of the local uplift and more deeply buried. The Chilwa alkaline magmatism post-dates the Karoo volcanism and has no apparent direct relationship to the Karoo magmatism. It seems most likely that the Chilwa magmatism was caused by crustal extension, which ruptured pre-existing zones of weakness and lead to decompressional melting.

The petrochemistry of the northern part of the Chilwa alkaline province, Malawi

Summary The Chilwa province of alkaline igneous rocks and carbonatites lies at the southern end of the East African rift, and is unique within the rift for its essentially intrusive nature. The province comprises numerous carbonatite centres (some with nepheline syenite and nephelinite), large complexes of nepheline syenite and syenite, and plutons of peralkaline syenite, quartz syenite and granite, together with dykes compositionally equivalent to all the major rock types. 196 rock samples from the northern half of the province have been chemically analysed and the data indicate that three rock series are present: (1) syenite-quartz syenite-granite, (2) nepheline syenite-syenite and (3) nephelinite-carbonatite-nepheline syenite. It has not proved possible to relate these series to a single parental magma, and it is concluded that three parental magmas of trachytic, phonolitic and nephelinitic-carbonatitic composition were involved. A model is suggested for the generation of these three magmas involving production of a zoned metasomatized lithosphere wedge by lithosphere focussing. Fracturing of the crust above the growing wedge led to pressure release and to the uprise of geotherms and volatiles with consequent partial melting at different levels throughout the metasomatized wedge. The metasomatism may have reached the base of the crust and it was from this region that trachyte magmas were produced by melting of rocks similar to syenitic fenites. Phonolitic magmas were generated directly at intermediate levels, and nephelinites were generated at the base of the metasomatized lithosphere.

A multi-method analysis of Si-, S- and REE-rich apatite from a new find of kalsilite-bearing leucitite (Abruzzi, Italy)

Abstract The crystal chemistry characteristics of a hydroxyl-fluor apatite from a recently discovered kalsilite-bearing leucitite from Abruzzi, Italy, were investigated by electron microprobe, single crystal X-ray diffraction, IR, Raman and micro-Raman spectroscopy. The apatite has exceptionally high S and relatively high Si, Sr and LREE, whereas the HREE content is negligible. The IR spectra confirm the presence of OH calculated from formula difference. A high positive correlation between Ca-site Substitution Index (CSI = 100(10-Ca)/Ca) and Tetrahedral Substitution Index (TSI = 100 (Si+C+S)/P atom/a.p.f.u.) and a systematic parallel increase in REE, S and Si indicate two substitution mechanisms, i.e. REE3+ + Si4+ = Ca2+ + P5+ and Si4+ + S6+ = 2 P5+. Site occupancy data and bond lengths, determined from structural refinements on selected samples, demonstrate that LREE and Sr show a marked preference for the Ca2 site, even though in the LREE-rich samples a partial substitution of LREE for Ca in the Ca1 site was observed. Tetrahedral distances (from 1.535 to 1.541 Å) reflect the substitution of Si4+ and S6+ for P5+, which is also confirmed by vibrational spectra. As (SiO4)4- and (SO4)2- substitute for (PO4)3-, the relative intensity of v1 Raman bands of (SO4)2- (at 1007 cm-1) and (SiO4)4- (at 865 cm-1) increase systematically, while that of phosphate decreases and the five components of phosphate v3 modes disappear. Moreover, the (PO4)3- Raman peak broadening is linearly correlated with the Si and S concentrations. Apatite crystals are sometimes zoned with compositions varying from SiO2 = 1.15-2.07 wt.%, ∑(LREE2O3) = 0.56-1.08 wt.% and SrO = 0.58-1.02 wt.% in the core to 3.98-5.03, 4.14-6.73 and 1.97-2.17, respectively, in the rim. A sharp, strong enrichment in Sr and LREE in the rim indicate that the apatite suddenly became an acceptor of these elements in the late stages of crystallization.

The crucial role of lithospheric structure in the generation and release of carbonatites: geological evidence

Abstract A recent database and world distribution map of carbonatites supports previous observations of the spatial and temporal aspects of these rocks, and provides new observations that are important for understanding their petrogenesis. These data reveal that there is an overwhelming concentration of carbonatites in Precambrian cratonic areas, most of which are elevated topographically. Thus, although approximately two-thirds of carbonatites are Phanerozoic in age, at least 88% of all dated carbonatites are located in the cratons, demonstrating a remarkable tendency for a Precambrian host. This observation suggests a link with kimberlites as diamond-bearing kimberlites are confined to the Archaean areas of cratons. The age data show that in many carbonatite-bearing provinces there has been repetition of carbonatite emplacement, with up to five episodes separated by hundreds of millions of years. In at least three provinces such activity extends from the late Archaean to relatively recent times and, because of the drift of the plates, this would seem to preclude any direct role for mantle plumes in carbonatite genesis. Magmatism is activated when lithosphere lesions are reopened in response to major changes in global plate movement patterns.

Extension of the melilite-carbonatite province in the Apennines of Italy: the kamafugite of Grotta del Cervo, Abruzzo

Abstract A new occurrence of a rare kamafugite near L’Aquila, Abruzzo, is described in detail to characterize its paragenesis and to establish possible genetic links with similar alkaline mafic igneous rocks from the Oricola-Camerata Nuova (OC) volcanic field, ~20 km to the west. Both occurrences belong to the Umbria-Latium-Ultralkaline-District (ULUD), an igneous district represented by rare kamafugites and carbonatites and distinct from the much more voluminous Roman Region (RR) rocks. The new kamafugite was found in a cave known as Grotta del Cervo (GC), associated with epiclastic and pyroclastic rocks. In the latter, lapilli ash tuff, welded lapilli, ultramafic xenoliths, cognate lithics and pelletal lapilli have been identified. The mineralogy of the welded lapilli comprises, in order of decreasing abundance, diopside, leucite, haüyne, Mg-mica, andraditic garnet, apatite, magnetite, kalsilite andolivine. The rock is carbonate-free. Basedon bulk-rock chemistry it is classified as a kamafugite, closely approaching the composition of ULUD kamafugites, according to Sahama’s (1974) criteria. Separate lapilli ash tuff, characterized by the same silicate mineralogy as that of the welded lapilli, plus modal carbonate exceeding 10 wt.%, is classified as a carbonatitic kamafugite. Bulk-rock andtrace-element compositions confirm that the Grotta del Cervo rocks closely approach the ULUD analogues. The Grotta del Cervo occurrence partially fills the geographical and compositional gap between ULUD rocks andthe rocks from the Vulture Complex, also a carbonatite and melilitite locality ~200 km south of GC, and adds considerably to the bulk of kamafugitic and related rocks lying along the Italian Apennines. The petrogenesis of these kamafugites rocks is discussed and possible mineralogical similarities with the Roman Region rocks are highlighted.

Carbonatite lapilli-bearing tuff and a dolomite carbonatite bomb from Murumuli crater, Katwe volcanic field, Uganda

Abstract A group of carbonate-rich tuffs are described from the Murumuli crater, Katwe-Kikorongo volcanic field, SW Uganda which contain abundant carbonatite pelletal lapilli, together with melilitite lapilli and a range of xenocrysts and lithic fragments including clinopyroxenites considered to be of mantle origin. The carbonatite lapilli consist essentially of Sr-bearing calcite and Mg-calcite which form quenchtextured laths. The lapilli contain microphenocrysts of Ti-magnetite, perovskite, apatite, clinopyroxene, sanidine and altered prisms of melilite. A 7 cm long dolomite carbonatite bomb is described which displays a form typically assumed by lava clots erupted in a molten state. Chemical analyses of a tuff, the bomb and a range of minerals are presented. Carbonatite clearly played an important role in the Katwe-Kikorongo magmatism and it is suggested that carbonatite magma evolved from carbonate-bearing melilitite.

Compositional evolution and cryptic variation in pyroxenes of the peralkaline Lovozero intrusion, Kola Peninsula, Russia

Abstract The Lovozero alkaline massif is the largest of the world’s layered peralkaline intrusions (~650 km2). We describe the evolution of clinopyroxene from the liquidus to the late residual stage throughout the whole vertical section (2.5 km thick) of the Lovozero Complex. Microprobe data (~990 analyses) of the clinopyroxenes define a relatively continuous trend from diopside containing 15-20% hedenbergite and 10-12% aegirine components, to pure aegirine. The main substitutions during the evolution of the Lovozero pyroxenes are (Na,Fe3+,Ti) for (Ca,Mg,Fe2+). The composition of the pyroxene changes systematically upwards through the intrusion with an increase in Na, Fe3+ and Ti and decrease in Ca and Mg. The compositional evolution of the Lovozero pyroxene reflects primary fractionation processes in the alkaline magma that differentiated in situ from the bottom to the top of the magma chamber as a result of magmatic convection, coupled with the sedimentation of minerals with different settling velocities. The temperature interval of pyroxene crystallization is very wide and probably extends from 970 to 450ºC. The redox conditions of pyroxene crystallization in the Lovozero intrusion were relatively low, approximating the QFM buffer.