Fertile Mantle Source Research Articles

Multiple mantle sources during island arc magmatism: U–Pb and Hf isotopic evidence from the Kohistan arc complex, Pakistan

ABSTRACT This paper presents high‐precision U–Pb ages and initial Hf isotopic compositions of zircon from mafic to felsic rocks of the Kohistan Arc Complex, Pakistan. Three magmatic pulses tapping geochemically different reservoirs are distinguished. Partial melting of mantle with MORB‐type isotopic characteristics generated 99–92‐Ma‐old magmas. Plutonism around 85 Ma tapped a more fertile mantle source, most likely consisting of a >600‐Ma‐old metasomatically enriched mantle, or of mantle contaminated by an old sedimentary component; 82‐Ma‐old felsic peraluminous dykes have MORB‐type isotopic compositions considered to be inherited from remelting earlier magmas in the deep base of the arc. The isotopic results demonstrate several and rather rapid changes in melt source region during arc development. They also show that there was subordinate continental influence and negligible importance of slab components for the Hf budget during the generation of the Kohistan Arc Complex.

The Round Mountain serpentinite mélange, northern Coast Ranges of California: An association of backarc and arc-related tectonic units

The Round Mountain serpentinite me lange, tectonically juxtaposed to the Franciscan Complex and to the Coast Range ophiolite, records structural and petrologic evidence for the tectonic processes that once affected late Mesozoic western North America. The melange mainly includes blocks of ultramafic and mafic compositions in a sheared ultramafic matrix. This matrix is thought to be essentially derived from shearing of highly serpentinized dunites, and more rarely harzburgites with or without clinopyroxene. Primary modal mineralogy of the ultramafic components, together with their spinel compositions and major and trace element chemistries, are consistent with both (1) backarc and (2) forearc and arc origins. Blocks of diabase, basaltic lavas, and rare gabbros display chemical characteristics similar to backarc magmas with specific Zr/Y ratios. The medium-(Zr/Y) group may be generated by ∼23% of incremental melting from a fertile-mantle source. Rocks related to alkalic magmatism plot in the high-(Zr/Y) group, whereas those influenced by arc processes correspond to the low-(Zr/Y) group. Our data suggest that before the onset of the east-dipping Franciscan subduction, a Middle Jurassic backarc basin slightly affected by alkalic and arc-related magmatic processes (recorded by the first stage of the Coast Range ophiolite) lay west of the ensimatic Smartville island arc that was later involved in the Sierra Nevada orogeny. Subsequent collapse of the backarc basin initiated Franciscan subduction. The Late Jurassic arc and forearc domains (recorded by the second stage of the Coast Range ophiolite) were built on a part of this trapped backarc lithosphere. The Round Mountain serpentinite melange was formed by underthrusting of the upper part of the backarc crust followed by its tectonic accretion to the remaining backarc and the forearc- and arc-related mantle sections of the overriding lithospheric wedge.

Ancient and Modern Subduction Zone Contributions to the Mantle Sources of Lavas from the Lassen Region of California Inferred from Lu-Hf Isotopic Systematics

Hafnium isotopic compositions have been determined on a suite of INTRODUCTION calc-alkaline and high-alumina-olivine tholeiitic lavas from the The geochemical and isotopic variability of mantle wedge Lassen region of California and are used, in conjunction with peridotites beneath continental volcanic arcs is poorly previously published mineralogical, geochemical, and isotopic data, constrained. This is a result of the fact that most of what to constrain their petrogenesis. Positive correlation between Hf is known about the geochemistry of the mantle wedge is values and geochemical indices of the modern subduction component inferred from the composition of basaltic arc lavas, which, indicates that the isotopic compositions of the calc-alkaline lavas although derived from the wedge, are affected by variable record addition of radiogenic Hf from the subducted slab. However, contributions from the subducting slab and continental the addition of the modern subduction component increases the Hf crust (e.g. Gill, 1981; Tatsumi et al., 1986; Hawkesworth values of most calc-alkaline lavas by <0·5 units over estimates of et al., 1993, 1994). Despite these limitations there is non-subduction enriched peridotites of the mantle wedge. The Lu–Hf significant evidence to suggest that mantle wedge periisotopic systematics of the Lassen lavas suggest that the calc-alkaline dotites are heterogeneous in composition and mineralogy. magmas have equilibrated with garnet at some point in their history, For example, it has been postulated that the compositions whereas the tholeiitic magmas have not. These observations require of mantle peridotites beneath the Cascade arc vary from the two lava types to be derived from different sources. The isotopic compositions that are similar to sources for mid-ocean variability of the Lassen lavas cannot be produced by mixing mantle ridge basalt (MORB), Hawaiian ocean-island basalt sources inferred to be present in the eastern–central Pacific and (OIB), and high-alumina-olivine tholeiites (HAOT) from western USA with a modern subduction component. Instead, the the Basin and Range province (Hughes, 1990; Leeman isotopic variability is consistent with mixing of a depleted mantle et al., 1990; Bacon et al., 1997; Borg et al., 1997; Clynne source, a more fertile mantle source enriched by an ancient subduction & Borg, 1997). The restite mineralogy of the source component, and a modern subduction component. regions for Cascade magmas is poorly constrained as well. For example, experiments conducted by Baker et al. (1994) found that calc-alkaline basalts from the Mt. Shasta region of California did not have garnet on their liquidus, and Reiners et al. (2000) were able to model

Petrochemical Investigation of the Antique Ophiolite (Philippines): Implications on Volcanogenic Massive Sulfide and Podiform Chromitite Deposits

Abstract: The Antique ophiolite, located in Panay island (west‐central Philippines), corresponds to several tectonic slices within the suture zone between the Philippine Mobile Belt (PMB) and the North Palawan Block (NPB). It includes dismembered fragments of a basaltic sequence, dominantly pillow‐lavas with minor sheet flows, rare exposures of sheeted dikes, isotropic gabbros, subordinate layered mafic and ultramafic rock sequences and serpentinites. Most of the ophiolite units commonly occur as clasts and blocks within the serpentinites, which intrude the whole ophiolitic body, as well as, the basal conglomerate of the overlying Middle Miocene sedimentary formation.The volcanic rock sequence is characterized by chemical compositions ranging from transitional (T)‐MORB, normal (N)‐MORB and to chemistry intermediate between those of MORB and island arc basalt (IAB). The residual upper mantle sequence is harzburgitic and generally more depleted than the upper mantle underlying modern mid‐oceanic ridges. Calculations using whole‐rock and mineral compositions show that they can represent the residue of a fertile mantle source, which have undergone degrees of partial melting ranging from 9‐22.5 %. Some of the mantle samples display chondrite‐nor‐malized REE and extended multi‐element patterns suggesting enrichments in LREE, Rb, Sr and Zr, which are comparable to those found in fore‐arc peridotites from the Izu‐Bonin‐Mariana (IBM) arc system. The Antique ultramafic rocks also record relatively oxidizing mantle conditions (Δlog fO2 (FMQ)=0.9‐3.5). As a whole, the ophiolite probably represents an agglomeration of oceanic ridge and fore‐arc crust fragments, which were juxtaposed during the Miocene collision of the PMB and the NPB. The intrusion of the serpentinites might be either coeval or subsequent to the accretion of the oceanic crust onto the fore‐arc.Volcanogenic massive sulfide (VMS) deposits occur either in or near the contact between the pillow basalts and the overlying sediments or interbedded with the sediments. The morphology of the deposits, type of metals, ore texture and the nature of the host rocks suggest that the formation of the VMS bodies was similar to the accumulation of metals around and in the subsurface of hydrothermal vents observed in modern mid‐oceanic ridge and back‐arc basin rift settings. The podiform chromitites occur as pods and subordinate layers within totally serpentinized dunite in the residual upper mantle sequence. No large coherent chromitite deposit was found since the host dunitic rocks often occur as blocks within the serpentinites. It is difficult to evaluate the original geodynamic setting of the mineralized bodies since the chemistry of the host rocks were considerably modified by alteration during their tectonic emplacement. A preliminary conclusion for Antique is that the VMS is apparently associated with a primitive tholeiitic intermediate MORB‐IAB volcanic suite, the chemistry of which is close to the calculated composition of the liquid that coexisted with the podiform chromitites.

Mantle compositional control on the extent of mantle melting, crust production, gravity anomaly, ridge morphology, and ridge segmentation: a case study at the Mid-Atlantic Ridge 33–35°N

Abstract Mantle temperature variation and plate spreading rate variation have been considered to be the two fundamental variables that determine the extent of mantle melting and ocean crust production. Along the length of a ∼200 km portion of the Mid-Atlantic Ridge (MAR) between the Oceanographer (35°N) and Hayes (33°N) transforms, the mantle potential temperature is the same, the plate spreading rate is the same, but the extent of mantle melting and crustal production vary drastically. In addition to the typical crustal thickness variation on ridge segment scales at the MAR, i.e. thicker at segment centers and thinner at segment ends, there exist between-segment differences. For example, the ∼90 km long segment OH-1 is magmatically robust with a central topographic high, thick crust, and a large negative gravity anomaly whereas the ∼45 km long segment OH-3 is magmatically starved with a deep rift valley, thin crust and a weak negative gravity anomaly. We demonstrate that the observed differences in the extent of mantle melting, melt production and crustal mass between segments OH-1 and OH-3 are ultimately controlled by their fertile mantle source compositional difference as reflected by the lava compositional differences between the two segments: >70% of OH-1 samples studied (N=57) are enriched MORB with [La/Sm]N>1, but >85% of OH-3 samples studied (N=42) are depleted MORB with [La/Sm]N

Metabasites along the amphibolite-granulite facies transition in the Okiep Copper District, South Africa

At least two types of metabasites occur along the granulite amphibolite facies transition in the Okiep Copper District in western Namaqualand. Low-pressure mafic granulites are mainly found in metagranitoids (Nababeep Gneiss) and supracrustal sequences (Khurisberg Subgroup) in the southern granulite facies Okiep domain; they include low-MgO as well as Mg-rich ol-normative types. Prograde amphibolites and rare relics of mafic granulite occur in orthogneisses of the −1.8 Ga Gladkop Suite and associated volcano-sedimentary rocks of the Lammerhoek Subgroup in the northern Steinkopf domain. All metabasites were metamorphosed during the late Narnaqua (1.02 to 1.03 Ga) high-grade event and are of orthomagmatic origin: precursors of mafic granulites were of subalkaline tholeiitic, while those of amphibolites of transitional alkali olivine basaltic to tholeiitic composition. MORB-normalised spidergrams and REE patterns are vely similar for the two groups and indicate prominent large ion lithophile (LIL) element and LREE enrichment compared to MORB ((La/Lu)N mafic granulites, 1.2–16.0; amphibolites, 0.6–5.1). Geochemical differences were, however, confirmed for LIL elements; the lower and more erratic concentrations of K2O, Na2O, Rb, and Pb in the mafic granulites may be the result of mobile behaviour of LIL elements during high-grade metamorphism. However, observed increased LREE concentrations cannot be explained with post-magmatic mobilisation alone. REE modelling demonstrates that some REE patterns of mafic granulites may be explained with about 20 % of partial (batch) melting of a fertile mantle source with subsequent variable degrees of fractional crystallisation of clinopyroxene, olivine, plagioclase ± hornblende; higher melting rates up to 40 % need to be assumed for successful modelling of the Mg-rich granulite. 10–15 % partial melting of fertile mantle can reproduce the most primitive amphibolite REE patterns. However, partial melting of a primitive mantle source and subsequent fractionation could not reproduce LREE-rich patterns documented for most amphibolites; an enriched mantle source is indicated. Geochemical data exclude an intra-plate or a depleted MORB-type mantle source for the various types of mafic granulites as well as for amphibolites but do not allow discrimination between extensional (e.g. continental flood basalt) and subduction-related (e.g. arc type) geotectonic settings. Our data establish strong geochemical similarities to older 1.65 Ga amphibolites from central Bushmanland.

Major element heterogeneity in the mantle source of the North Atlantic igneous province

High-MgO (>8.5 wt%), aphyric lavas erupted at various locations in the North Atlantic igneous province are utilized to characterize the nature of mantle melting during the formation of this province. Based on the observation that the Ni concentration in residual mantle olivine mostly falls in the range of 2000–3500 ppm, these high-MgO samples are corrected for olivine fractionation until the Ni concentration of equilibrium olivine reaches 3500 ppm, to estimate the composition of primary mantle-derived melt. Estimated primary melt compositions suggest that this province is characterized by significant major element source heterogeneity possibly resulting from basalt addition prior to melting. Primary melts for Southwest Iceland and Theistareykir (North Iceland) are shown to require different source mantle compositions. Whereas the Theistareykir primary melt may be explained by the melting of pyrolitic mantle, the source mantle for Southwest Iceland must be enriched in iron, having molar Mg/(Mg+Fe), or Mg#, <0.88. This compositional dichotomy in Iceland seems to continue to adjacent Mid-Atlantic Ridge segments, i.e. the Kolbeinsey and Reykjanes Ridges. The primary melts for East and Southeast Greenland also indicate a fertile mantle source, and the estimate of Mg# is the lowest for the East Greenland source mantle (<0.87). The inferred spatial extent of source heterogeneity suggests the presence of a long-lived compositional anomaly in this igneous province since the opening of the North Atlantic.

ORIGIN OF POST-VARISCAN GABBRO-DERIVED GRANULITES (NORTHERN APENNINES, ITALY): CRUSTAL CONTAMINATION PROCESSES AND IMPLICATIONS FOR MANTLE SOURCES

The Late Cretaceous sedimentary melanges from the External Liguride Units of Northern Apennines include large slide-blocks of subcontinental mantle peridotites, MORbasalts and lower and upper continental crust rocks. The slide-block association has been interpreted as representative of a continent-ocean transition between the Internal Liguride oceanic domain (Late Jurassic Western Tethys) and the thinned continental margin of the Adria plate (Marroni et al., 1998). The slide-blocks of lower continental crust consist of mafic and felsic granulites, which locally preserve primary contacts. The mafic granulites commonly display a metamorphic layering, but undeformed rocks preserving a gabbroic fabric are locally found. Undeformed mafic granulites are mostly represented by spinel-bearing gabbronorites, usually containing significant amounts of either olivine or Fe-Ti-oxides. Olivine- and Fe-Ti oxide-bearing rocks locally show spinel-pyroxene symplectites and garnet coronas, respectively. The felsic granulites are mainly quartzo- feldspathic rocks consisting of mesoperthitic to perthitic feldspar, quartz and garnet. The gabbroic protoliths of the granulites were emplaced at about 290 Ma at deep crustal levels, where they underwent slow cooling and recrystallisation under granulite-facies conditions (P = 0.7-0.8 GPa, T = 800-900°C). They were exhumed to upper levels, in association with the felsic granulites, in late Triassic-middle Jurassic times. The gabbro-derived granulites can be recognized as cumulus rocks with negligible amounts of residual trapped liquid, on the basis of low SiO2/Al2O3 ratios and overall low contents of incompatible trace elements. The Mg# value ranges from 80 to 52, and point to negative correlations with TiO2 and MnO, thus indicating a tholeiitic differentiation trend. Most gabbro-derived granulites have slightly LREEenriched patterns showing decreasing Eu positive anomaly with increasing total REE abundances. Chondrite normalization of incompatible trace elements reveals spikes at Ba and Sr, and a slight Zr depletion. The quartzo-feldspathic granulites have LREE enriched patterns, with nearly flat HREE and no or slightly positive Eu anomaly; Ba is abruptly enriched relative to REE, whereas Nb and Ti are depleted. The gabbro-derived granulites show a wide range in Sr and Nd isotopic compositions. The Sr isotopic ratio recalculated at 290 Ma varies between 0.7031 and 0.7077, and the initial eNd ranges between +6.8 and -4.5. Two samples of quartzo-feldspathic granulite yield age-corrected Sr isotopic ratios of 0.7107 and 0.7109, and eNd of -8.0 and -5.7. As a whole, the Nd and Sr isotopic data at 290 Ma form a hyperbolic array, in which the olivine-bearing gabbronorites have the highest eNd values and the lowest Sr isotopic ratios. Clinopyroxenes have been analyzed for trace elements by ion microprobe. Clinopyroxene from olivine-bearing gabbronorites shows peculiar compositions that indicate a metamorphic origin through olivine-plagioclase reaction, i.e. the igneous protoliths of the olivine-bearing gabbronorites were most likely troctolite-type cumulates. Clinopyroxenes from Fe-Ti oxide bearing gabbronorites show igneous geochemical trends, thus suggesting that these rocks contained clinopyroxene as original igneous phase. Petrography, bulk-rock and mineral composition indicate that the gabbro-derived granulites can be related to a fractional crystallization process, with early separation of olivine and plagioclase, followed by the replacement of olivine by pyroxene at the liquidus. Trace element modelization of the parental liquid compositions applied to the olivine-bearing rocks yields LREE- and LILE-enriched liquids, with absence of negative Nb anomaly, similar to plume-type MOR-basalts and continental tholeiites. However, a P-MORB origin seems to contrast with the initial Nd and Sr isotopic compositions, which are close to depleted mantle values at the time of emplacement. AFC modelization was successfully applied to obtain the isotopic compositions of the most contaminated samples, starting from the trace element and isotopic compositions of the parental liquids of the olivine gabbronorites and assuming a crustal contaminant with low Sr/Nd and isotopic composition comparable to that of the quartzo-feldspathic granulites. AFC calculations also indicate that the parental liquids of the olivine-bearing gabbronorites cannot be ascribed to N-MORB primary liquids. The primary mantle magma was necessarily characterized by moderate LILE enrichment, although an increase in LILE concentrations could have been enhanced by a small crustal contribution. The LILE enrichment in the parental liquids of the gabbro-derived granulites may be explained with a low degree partial melting of a rather fertile lithospheric mantle source. Alternatively, the primary liquids of the gabbro-derived granulites were related to a mantle source enriched in LILE as a result of the Variscan subduction event.

The petrogenesis of the eastern Pyrenean peridotites: an integrated study of their whole-rock geochemistry and Re-Os isotope composition

The Re-Os isotopic compositions of sixteen peridotites from the eastern Pyrenean ultramafic massifs have been determined in conjunction with their whole-rock major, trace, and chalcophile element contents. The data suggest that the peridotites represent residues after the extraction of up to 25% partial melt from a fertile mantle source at relatively high pressures (>20 kbar). Linear correlations between the incompatible-element contents of the peridotites and indices of major element depletion require that the initial melt extraction must have been followed by metasomatism by an incompatible-element enriched component. The S contents of the majority of the peridotites show a linear correlation with major element depletion, consistent with the extraction of a S-saturated basaltic melt from a fertile peridotite source. Low Cu/S ratios in a number of samples, however, suggest that parts of some of the massifs may have been infiltrated by an exotic sulphide component. The range of Os abundances in the peridotites is relatively limited (3.13–4.74 ppb) and unrelated to either the major or trace element contents of the peridotites. In contrast, the Re abundances vary widely (65–497 ppt) and correlate with their degree of major element depletion. Rhenium, sulfur, and Al 2O 3 are all positively correlated, suggesting all are controled by melt extraction. The 187Os/ 188Os ratios of the peridotites range from 0.1157 to 0.1294 and show good correlations with whole-rock Re, S, and Al 2O 3 abundances. From the y-intercept of the 187Os/ 188Os-S trend an initial 187Os/ 188Os ratio of ∼0.1159 is calculated for the peridotites, yielding a model age of 1.9 ± 0.3 Ga for the isotopic homogenisation of the mantle lithosphere that underlies the massifs. This model age is similar to the age of major crustal growth in Europe and implies a broadly synchronous stabilisation of both crust and lithospheric mantle at this time. The linear decrease of Re with degree of melt extraction is consistent with nonmodal melting of a Re-bearing base-metal sulphide phase. A sulphide-silicate melt partition coefficient (D sul/sil Re) of ∼325 is calculated for Re, indicating that the chalcophile behaviour of Re during melting may be similar to that of the moderately chalcophile elements, such as Cu rather than the highly chalcophile Pt-group elements.

Geochemistry and tectonomagmatic affinity of the Misheguk massif, Brooks Range ophiolite, Alaska

Petrologic and geochemical data from the type location of the Misheguk igneous sequence (Misheguk massif), clarify the tectonomagmatic origin of the Brooks Range ophiolite. At Misheguk the igneous sequence consists, in ascending order, of (1) mantle peridotite (2) ultramafic cumulates (3) layered gabbro (4) massive gabbro and high level intrusives and (5) out of sequence intrusives. The various units are mostly transitional except for local intrusive and fault relations. Sheeted dikes and volcanic cover sequences associated with Brooks Range ophiolite have not been found at Misheguk, but are documented at the Siniktanneyak body. Distinctive compositional features found at Misheguk and other bodies throughout the ophiolite include a dunite dominate mantle sequence depleted in Ti, Al and Ca, and enriched in Cr and Ni. This sequence is transitional with cumulates with a crystallization sequence of olivine-clinopyroxene-plagioclase that produces a succession of dunite, wehrlite and olivine gabbro. The cumulate sequence is interpreted to reflect source characteristics of melts that are transitional between MORB- and arc-types. Mineral chemistry comparisons show that average compositions of olivine of Fo 90.7, orthopyroxene with 0.5-2.7% Al 2O 3, and Cr-rich spinel with average Cr ∗ of 0.58 are intermediate between those from MORB- and arc-type mantle peridotites. Coexisting cumulate olivine and plagioclase compositions are similar to those found in dunite-rich arc complexes. Cr Mg abundances in spinel from various mantle peridotites cluster in distinct groups that reflect various degrees of partial melting. Models of partial melting paths based on Cr Ti abundances in mantle peridotite predict the mantle sequence experienced around 20% melt removal from a fertile (Ligurian) mantle source. These geochemical characteristics are similar to those predicted for, and documented in interarc basins.

Arc and back-arc geochemistry in the southern Kermadec arc-Ngatoro Basin and offshore Taupo Volcanic Zone, SW Pacific

Abstract Back-arc basin basalts from the Ngatoro Basin (the southern end of the Havre Trough) are similar geochemically to, yet subtly distinct from, basalts of the Havre Trough to the north. Whole rock and glass chemistry are consistent with derivation from a fertile mantle source with subsequent evolution by fractionation of olivine (+ Cr-spinel) + plagioclase, and then clinopyroxene. Basalts from the vicinity of Rumble IV seamount at the southern end of the Kermadec island arc, and the eastern Ngatoro rift escarpment, are strongly porphyritic relative to the back-arc basin basalts and show trace element (high LIL abundances and highly depleted HFS abundances) and isotopic signatures of subduction zone basalts. At its southern end, the Ngatoro Basin penetrates the continental crust of New Zealand creating a major, 3000 m deep bathymetric re-entrant in the slope-break; the slope-break marks the transition from oceanic to continental crust. Basalts from the floor of the Ngatoro Basin re-entrant are isotopically distinct from the basalts of the oceanic sector in that they have higher Sr and correspondingly lower Nd isotope ratios and are comparable to basalts of the Taupo Volcanic Zone (TVZ) to the south. In contrast to the basalt-dominated oceanic sector, basalts from the offshore TVZ, a 100 km long area extending roughly NNE from White Island to the submarine Whakatane arc volcano at the edge of the continental slope-break, occur in association with andesites, dacites and rhyolites. These basalts are generally strongly porphyritic (olivine + plagioclase + clinopyroxene) and show trace element abundances typical of suprasubduction zone rocks. However, offshore TVZ basalts show subtle distinctions from onshore TVZ basalts to the south; the former have more radiogenic Sr isotopes. Furthermore, their high field strength element and transition element systematics appear to overlap with those of basalts from the Kermadec arc to the north. The authors attribute these lateral (along arc) and transverse (across arc) variations to source heterogeneity and variable fertility in the sources of the arc and back-arc basin magmas. Sources of the arc-front magmas are more refractory and also more susceptible to contamination by slab-derived fluids than sources for back-arc basin magmatism, reflecting the dynamic nature of flow from the back-arc into the mantle wedge beneath the volcanic front.

The ultrapotassic rocks: Characteristics, classification, and constraints for petrogenetic models

A definition for ultrapotassic rocks is introduced using the chemical screens K 2O > 3 wt.%, MgO > 3 wt.% and K 2O/Na 2O > 2 for whole-rock analyses. On this basis a data base of more than 800 analyses was retrieved by a literature review. These are grouped according to a ‘resemblance’ classification designed to aid petrogenetic modelling by by-passing mineralogically based nomenclature groupings. Three major chemical end-member groups are recognised: Group I, lamproites, characterised by low CaO, Al 2O 2 and Na 2O, and high K 2O/Al 2O 3 and Mg-number. Group I rocks have the highest ‘trace’ incompatible element contents, and carry mostly depleted dunite and harzburgite mantle xenoliths. West Kimberley and Gaussberg are chosen as the standard members. Group I rocks usually have no associated non-ultrapotassic rocks, but non-standard members have calc-alkaline and shoshonitic associates, may occur in orogenic areas, and frequently have some chemical characteristics which are transitional to those of group III. Group II, known as kamafugites, have Toro Ankole rocks as their standard members. They have low SiO 2 and high CaO. Incompatible elements are less enriched than group I, but have a positive Sr spike. Group III rocks occur in orogenic areas and have high CaO and Al 2O 3. Mg-number is often low due to fractional crystallisation, but primary magmas have many of the characteristics of mantle derivation. Partial melting of mantle material previously enriched in incompatible elements is considered the most likely explanation for the origin of primary magmas for all three groups. Processes in the mantle before enrichment, during the enrichment event, and during partial melting at the magma genesis stage are considered separately for their likely effect on magma characteristics defined by the data base. Group I rocks probably originate from a depleted mantle under H 2O- and F-rich, CO 2-poor and possibly CH 4-rich conditions. The range in silica contents of primary lamproitic magmas may be due to partial melting at variable depth, with the diamond-bearing olivine lamproites originating from the greatest depth. The low SiO 2, high CaO and Sr of group II rocks suggests melting in a CO 2-rich, H 2O-poor environment, which is also indicated by CO 2-rich volcanic gases. The source for Toro Ankole lavas may originally have been depleted, but evidence from nodules is lost due to pervasive metasomatism. Group III rocks occur in orogenic areas, and many of their chemical characteristics must be explained by similar processes to those in less alkaline island-arc magmatism. The low Ti, Nb and Ba may be due to titanate phases being stable in the residue during partial melting, or to reactions in the subducted slab. High K 2O may be derived from subducted crustal material or from mantle enrichment occurring beneath the maturing arc, but unrelated to the subducted slab. High CaO and Al 2O 3 are favoured by melting a fertile mantle source. A fourth, transitional group includes both rocks which are likely to be mantle-derived and those which may be cumulates from crustal melts. A number of minettes may be due to crustal modification of mantle-derived potassic magmas.

Proterozoic Mantle Heterogeneity: Geochemical Evidence from Contrasting Basic Dykes

Summary Two swarms of early Proterozoic basic dykes occur in the Archaean craton of SW Greenland. The MD dykes are tholeiitic dolerites, while the BN dykes are norites. Virtually identical norites occur with tholeiitic dolerites in the Scourie swarm of NW Scotland and in the Laramie, Bighorn and Beartooth ranges of Wyoming, USA. Close geochemical similarities between typical coarse-grained norites and quenched varieties in SW Greenland and Wyoming suggest that the former type has not been strongly modified by crystal accumulation. The mineralogy and geochemistry of the quenched norites closely resemble those of boninites. Most of the norites have high MgO ( c. 13–18 wt%) and correspondingly high Cr and Ni, but they also have relatively high silica, alkali and large ion lithophile (LIL) element abundances. They are all light rare-earth element (REE) enriched (La = 40 times average CI chondrite, La N /Lu N c. 7). In contrast, most of the tholeiitic MD dolerites have fiat, unfractionated REE distributions. The tholeiites were probably derived from a fertile mantle source. However, the norites originated from a depleted harzburgitic mantle which had earlier borne Archaean komatiitic and tholeiitic magmas but which had been metasomatically replenished in LIL elements by fluids derived from Archaean oceanic and lower continental crustal material returned to the mantle during late-Archaean crustal thickening. Some major layered basic igneous intrusions, such as Bushveld and Stillwater, were derived by the mixing of two different basic magmas, which were equivalent to the tholeiitic and noritic (boninitic) dyke magmas of SW Greenland, NW Scotland and Wyoming. The apparently ubiquitous occurrence of tholeiite-norite associations suggests that similar large-scale mantle heterogeneities, due to the voluminous extraction of Archaean komatiitic and tholeiitic magmas, had become well established by the early Proterozoic.

Petrogenesis of some Ligurian peridotites—I. Mineral and bulk-rock chemistry

Bulk-rock and mineral compositions of eleven lherzolitic specimens have been determined. Eight samples are from the Erro-Tobbio thrust sheet of the Gruppo di Voltri, western Liguria; three samples are from allochthonous eastern Ligurian masses within the External Ligurides. All specimens consist of spinel (±plagioclase) in addition to ol + opx + cpx, and have been derived from mantle portions of the South Alpine-Apennine lithospheric plate. Electron microprobe analyses of 11 olivines, 17 orthopyroxenes, 13 clinopyroxenes, eight spinels and one plagioclase have been performed. Except for Ca-rich and Ca-poor lamellae in some pyroxenes, and iron-enriched rims in a few spinels, the analyzed phases are remarkably homogeneous. Mineral compositions vary insignificantly from rock to rock. Consideration of major element partitioning in coexisting opx + cpx pairs, combined with the petrogenetic grid for aluminous lherzolite bulk compositions, yield the following provisional P- Tequilibration conditions: Erro-Tobbio complex, 1150 ± 50° C, 16 ± 6 kbar; External Ligurides, 1000 ± 50° C, 14 ± 6 kbar. Evidently the western Ligurian peridotites equilibrated at higher temperatures and possibly at slightly greater depths with the upper mantle than those from eastern Liguria. The lherzolite samples have nearly identical bulk-rock major element compositions. Comparisons with estimated compositions for the upper mantle, and for model pyrolite indicate that these Ligurian peridotites are primitive or only very slightly depleted in basaltic constituents. Thus, based on major element chemistry, the Ligurian peridotites cannot be regarded as residua left after extensive partial fusion, hence evidently cannot have produced large proportions of oceanic tholeiite liquid; on the other hand, they seem to represent a fertile mantle source for such melts. Relics of garnet have not been found, and apparent temperatures for both Erro-Tobbio and eastern Ligurian ultramafic suites exceed the classical intraplate oceanic geotherm; accordingly, these bodies may represent rising mantle material that recrystallized during ascent in the general vicinity of a spreading center, without having undergone extensive partial melting.