M. A. Morrison

Elemental fingerprints of isotopic contamination of hebridean Palaeocene mantle-derived magmas by archaean sial

One of the major puzzles presented by the geochemistry of the Palaeocene plateau lavas of Skye and Mull (N.W. Scotland) is that, although a very strong case can be made that the magmas are variably isotopically contaminated by Archaean Lewisian continental crust, little evidence has been gleaned to date from their major- and trace-element compositions to illuminate this hypothetical process. The combined results of published Sr-, Nd- and Pb-isotope studies of these lavas allow the basalts and hawaiites to be divided into three broad groups: essentially uncontaminated; contaminated with granulite-facies Archaean crust; contaminated with amphibolite-facies Archaean crust. Members of each group show distinctive chondrite-normalised incompatible-element patterns. The processes which gave rise to isotopic contamination of these lavas also affected the abundances and ratios of Ba, Rb, Th, K, Sr and light REE in the magmas, whilst having negligible effects on their abundances and ratios of Nb, Ta, P, Zr, Hf, Ti, Y and middle-heavy REE. Because such a wide range of elements were affected by the contamination process, it is postulated that the contaminant was a silicate melt of one or more distinctive crustal rock types, rather than an aqueous or similar fluid causing selective elemental movements from wall rocks into the magmas. As previous experimental and isotopic studies have shown that the Skye and Mull basic magmas were not constrained by cotectic equilibria at the time when they interacted with sial, the compositions of the contaminated lavas have been modelled in terms of simple magma-crust mixtures. Very close approximations to both the abundances and ratios of incompatible elements in the two groups of contaminated basalts may be obtained by adding 15% to 20% of Lewisian leucogneisses to uncontaminated Palaeocene basalt. Nevertheless, major-element constraints suggest that the maximum amount of granitic contaminant which has been added to these magmas lies between 5% and 10%. These estimates may be reconciled by postulating that the contaminants were large-fraction cotectic partial melts of Lewisian leucogneisses, leaving plagioclase residua. A corollary of this hypothesis is that it is necessary to postulate that the “magma chambers” where the sialic contamination occurred were, in fact, dykes or (more probably) sills. The very large surface-to-volume ratios of such magmas bodies would permit the systematic stripping, by partial melting, of the most-easily-fusible leucogneisses and pegmatites from the Lewisian crust, whilst failing to melt its major rock types. A present-day analogue to this situation may be the extensive sill-like magma bodies detected by geophysical methods within the continental crust beneath the Rio Grande Rift, southwestern U.S.A.

Asthenospheric and lower-lithospheric mantle contributions to continental extensional magmatism: An example from the British Tertiary Province

Abstract The compositions of Palaeocene basic magmas in Skye and Mull, British Tertiary Igneous Province (BTIP), shed light on the general problem of the relative importance of lithospheric and asthenospheric mantle as sources of continental magmatism during extension. After the effects of crustal contamination in some magmas have been detected, and the samples showing it eliminated from consideration, the remainder show a significant diversity of trace-element compositions. Basic lavas from Skye and Mull which lack crustal contamination have contents of compatible and moderately-compatible elements (e.g., Ni, Cr, Y, Yb) and also of moderately-incompatible elements (e.g., Ti, P, Zr, Hf, Sr, MREE) which are within the range shown by the tholeiitic and mildly-alkalic ocean-island basalts (OIB), as from Iceland. In contrast, their contents of strongly- and very-strongly-incompatible elements (e.g., K, Rb, Ba, U, Th, Nb, Ta, LREE) are within the range shown by normal mid-ocean-ridge basalts. These BTIP magmas are interpreted as second-stage melts from mantle which had already lost a very small melt fraction. The latter is thought to be represented by the strongly-alkalic Permo-Carboniferous dyke swarms associated with rifting at that time in NW Scotland. Residual uppermost asthenosphere from the late Palaeozoic magma genesis event is postulated to have become attached to the thickening lithosphere beneath NW Scotland and thus to have remained coupled to it throughout the Mesozoic, until reactivated by Palaeocene stretching. Occasional hawaiites in the top third of the Skye lava pile remnant, and also basalt dykes of the swarm cutting the granites of the central complex (the last significant magmatic event at this centre) have the elemental characteristics of typical North Atlantic OIB - notably La Ta =10–12 . These compositions are thought to demonstrate input from asthenospheric-source basic magmas throughout the lithospheric-source-dominated Skye and Mull magmatism. It is suggested that all the basic magmas in this province are ultimately derived from asthenospheric sources, but that the larger magma batches became contaminated during uprise by melting and assimilating the most-fusible parts of overlying lithospheric mantle (followed by lower crust in many cases). Smaller magma batches traverse the overlying lithosphere with little or no contamination. Perhaps the difference between the two lies in their flow behaviour - laminar or turbulent - during uprise.

Strongly potassic mafic magmas from lithospheric mantle sources during continental extension and heating: evidence from Miocene minettes of northwest Colorado, U.S.A.

Abstract Minette occurs as sparse dykes and sills in the Upper-Miocene Elkhead Mountains igneous province, NW Colorado. At the time of magma emplacement, the region was undergoing crustal extension and probably also heating of the sub-continental lithospheric mantle by the approaching Yellowstone plume. The predominant magmatism of the province comprises lava field remnants and hypabyssal plutons. Elemental variation within the minette suite is explicable in terms of fractional crystallisation which involved phlogopite separation from even the most magnesian (MgO = 10.68%) samples. Wide ranges of incompatible-element abundances and ratios occur in the minettes with MgO > 6.0%. Some of these ratios (e.g. Ti/Zr andLa/Nb) correlate well with 143 Nd/ 144 Nd in this suite. The minettes have a combination of relatively low values of both 87 Sr/ 86 Sr (0.70387–0.70413) and 143 Nd/ 144 Nd (0.51201–0.51227), with 206 Pb/ 204 Pb (17.28–17.47), 207 Pb/ 204 Pb (15.45–15.54) and 208 Pb/ 204 Pb (36.55–37.02). Taken together, these isotopic characteristics fall far outside the range of all oceanic igneous rocks and therefore rule out an exclusively asthenospheric source for the magmas. Genetic models involving crustal contamination of either basaltic (s.l.) or lamproitic liquids do not appear to explain satisfactorily the geochemical features of the minettes. Alternative models invoke either separate subcontinental lithospheric mantle sources for each magma batch or mixing between upwelling basaltic liquids and varying amounts of ultrapotassic lithospheric melts. Both models fit the geochemical data reasonably well but the latter is, in addition, consistent with a recent analysis by D. McKenzie [8] of the physical constraints on strongly potassic magma genesis during continental lithospheric extension and/or heating above a mantle plume. A brief survey of the tectonic settings of minettes and ultrapotassic rocks worldwide shows that a strong case can be made for their association in space and time with heating and/or thinning of sub-continental lithospheric mantle.

Quaternary volcanism in northwestern Colorado: Implications for the roles of asthenosphere and lithosphere in the genesis of continental basalts

Abstract Quaternary volcanic rocks were erupted at four locations in NW Colorado; Dotsero (4150 y.B.P.), Willow Peak (undated), McCoy (0.64 m.y. B.P.), and Triangle Peak (1.98-1.87 m.y.B.P.). At Triangle Peak, there are at least eleven lava flows, but eruptions at the other locations were monogenetic. Dotsero was the only hydrovolcanic eruption. The volcanic rocks are alkali basalts, containing the phenocryst assemblage: olivine, Fe-Ti oxide, ± clinopyroxene, ± plagioclase. The basalts are chemically similar to OIB, as would be expected from their intraplate crustal setting. Nevertheless, they have La/Ta, K/Ta, Ba/Ta and K/La ratios which are significantly higher than those of oceanic OIB. These differences cannot be explained by contamination during uprise of OIB-like basalt by continental crust of reasonable composition. It is, therefore, logical to assume that the Quaternary magmas contained a component of partial melt of subcontinental lithospheric mantle. This conclusion is in accord with the low 143 Nd/ 144 Nd ratios of the basalts. The geochemistry of the Quaternary basalts can be explained by mixing between three separate mafic magma end-member groups that were erupted in the same area during the Miocene. Group 1 magmas were OIB, representing partial melts of OIB-source asthenosphere. Group 2 magmas were minettes, with low 143 Nd/ 144 Nd ratios, regarded as partial melts of sub-Colorado lithospheric mantle. Group 3 magmas had high La/Ta ratios, and, generally, low LIL/HFS ratios. During the Miocene, the latter group of magmas are interpreted to have been derived by partial melting of asthenosphere that had been modified by subduction of oceanic lithosphere below the North America plate. The presence of a component of Group 3 magma in the Quaternary basalts indicates that the mantle source of this group was trapped for 8 m.y. in an uppermost asthenospheric layer which experienced very sluggish flow. We propose that this layer is equivalent to the thermal boundary layer — situated between the rigid part of the lithospheric mantle (above), and the convecting asthenosphere (below) — originally identified by calculations of the thermal histories of lithospheric plates.

Asthenosphere-derived magmatism in the Rio Grande rift, western USA: implications for continental break-up

Abstract Magmas that are generated at continental rift zones provide an insight into the processes operating during the early stages of continental break-up. Our detailed study of mafic volcanism along the axis of the Rio Grande rift shows that, throughout both phases (30–17 and < 13 Ma) of its evolution, magmas with compositions interpreted as melts from the asthenospheric mantle have reached the surface. This recognition of early phase (26 Ma) magmas with incompatible trace element concentrations and radiogenic isotope ratios resembling those normally associated with ocean-island basalts and small seamounts (OIB) is significant because: (1) magmas dominated by the composition of asthenosphere-derived melts are not usually thought to be characteristic of early-phase continental rifting; (2) Tertiary mafic magmatism of an age greater than late Miocene in Colorado and New Mexico was hitherto regarded as subduction-related. Previous studies have shown that the final erupted composition of asthenosphere-derived melts is determined by the potential temperature of the convecting mantle, the amount and rate of lithosphere extension, fractional crystallization and crustal contamination. However, in the Rio Grande rift and elsewhere, such as the Basin and Range province, Eifel, NW Sardinia and the Cameroon Line, the final composition of these melts is also significantly influenced by earlier magmatic episodes. During the initial stages of asthenosphere melt generation the magma batches that first penetrate may heat a previously undisturbed segment of lithosphere and mix with strongly potassic, low temperature melt fractions. When these segments have been subsequently temporarily purged of such fusible potassic fractions the asthenosphere-derived melts can rise unimpeded through the sub-continental lithosphere.

The Great Glen Fault : a major vertical lithospheric boundary

Two lamprophyre suites are used to constrain sub-continental lithospheric mantle domains in Late Caledonian Northern Britain (at 400 Ma). A Northern Highlands and a Southern domain are resolved. The first has low (ɛNd (−6.4 to − 12.8); the latter has higher (+3.9 to −3.4). The boundary between them is coincident with the surface expression of the Great Glen Fault. The two mantle domains tightly bracket the fault. The lamprophyre magmas were generated at depths of at least 100 km. At the end of the Caledonian Orogeny the Great Glen Fault was a major vertical discontinuity that transected the sub-continental lithospheric mantle.

Alkaline hybrid mafic magmas of the Yampa area, NW Colorado, and their relationship to the Yellowstone mantle plume and lithospheric mantle domains

The Yampa volcanic field (late Miocene) consists of about 70 outcrops of monogenetic cinder cones, lavas, dykes, volcanic necks and hydrovolcanic pyroclastic deposits and is situated in the most northerly part of the Rio Grande rift. Contemporaneous extension in this part of the rift was small, but there is geological and geophysical evidence that, by the late Miocene, the area was underlain by hot asthenosphere convected by the Yellowstone mantle plume. The Yampa rocks are mafic and chemically diverse, including basanites, alkali basalts, potassic trachybasalts, hawaiites and shoshonites. About half the rocks bear the xenocryst suite feldspar, pyroxene, Fe−Ti oxide, amphibole, biotite. There is a tendency for xenocryst-free rocks to be the most mafic, interpreted to indicate that the xenocrysts are cognate, and represent cumulate material from fractional crystallization of the magmas in deep crustal magma chambers. The elemental and isotopic (Nd and Sr) variations can be modelled by mixing variable proportions of partial melts of local lithospheric mantle with an OIB end-member formed by partial melting of asthenosphere. The OIB end-member appears to have the elemental and isotopic composition of typical Northern Hemisphere OIB, in particular the plume-derived basanites of Loihi seamount, Hawaii. The OIB end-member at Yampa is interpreted to have been derived from mantle convected in the Yellowstone mantle plume.

Geochemistry of mafic lavas in the early Rio Grande Rift, Yarmony Mountain, Colorado, U.S.A.

Abstract Mafic lavas (high-K basalts and shoshonites) erupted 21–24 Ma ago (early Miocene) near State Bridge, NW Colorado, and now exposed on Yarmony Mountain, were related to the initial phase of extension in the northernmost section of the Rio Grande Rift. The volcanism was of small volume, consisting of eleven flows on Yarmony Mountain, although much more voluminous contemporaneous lavas occur 50 km to the west. All eleven flows have been analysed comprehensively for major and trace elements, and five of the flows for Nd and Sr isotopes. Some samples experienced post-eruptive carbonation and leaching of alkali elements; the effects of these processes on normative compositions are examined, and it is suggested that major-element abundances are sufficiently well preserved to be fairly sure that the magmas experienced fractional crystallization at pressures appropriate for the mid-lower part of the crust. Most of the basalts are chemically closely related, and have the following characteristics: (1) they are depleted in Nb and Ta relative to LREE, a feature of volcanic arc magmas; (2) they have lower abundances of Rb and K, and to a lesser extent Ba and Th, for a given LREE content, than typical magmatic arc magmas; and (3) they have Nd and Sr isotopic ratios similar to both certain low - 143 Nd 144 Nd oceanic basalts, notably from the Kerguelen area, Indian Ocean, and subduction-related magmas from mature volcanic arcs, notably from the Andes, South America. We argue that the magmas erupted at Yarmony Mountain were generated by partial melting of asthenospheric mantle which had been modified by subduction of oceanic lithosphere during the Cenozoic. The relatively low abundances of the alkali elements appears to be a consequence of depletion of these elements in the mantle source by a previous episode of melt extraction, before that which generated the Yarmony magmas. An alternative is that the mantle source was depleted in alkali elements by a dehydration event, possibly involving a flow of CO2. One of the lavas in the Yarmony sequence is elementally and isotopically distinctive, having higher K, Zr, Ba, K 2 O Na 2 O , La/Ta and K/La, and lower 87 Sr 86 Sr than the rest of the samples. It is argued in detail that this is not the result of contamination by any reasonable crustal composition, and that this magma contained a component (∼10–20%) of ultrapotassic mafic magma derived by partial melting of subcontinental lithospheric mantle. There is no petrographic evidence for mixing, and the hybridization probably took place in magma chambers situated in the mid-lower crust.

Oligocene lamproite containing an Al-poor, Ti-rich biotite, Middle Park, Northwest Colorado, USA

Abstract A small 33 ±0.8 Ma lamproite pluton is exposed in the midst of a 23−26 Ma basalt-rhyolite province in Middle Park, NW Colorado. It contains abundant phlogopite phenocrysts in a fine-grained groundmass of analcime pseudomorphs after leucite, biotite, potassic fichterite, apatite, ilmenite and accessory diopside. The phlogopite phenocryst cores contain ~4 wt.% TiO2, 1% Cr2O3 and 0.2% BaO. The smallest groundmass biotites have normal pleochroism but compositions unlike any previously reported, with ~2% Al2O3, ~8% TiO2 and F <1.5%. Apart from those elements affected by leucite alteration, both the elemental and isotopic composition of this lamproite are close to those of the Leucite Hills, Wyoming. Its Nd-isotopic model age (TDM = 1.6 Ga) is outside the Leucite Hills range but within that of other Tertiary strongly potassic magmatism in the region underlain by the Wyoming craton. Evidence from both teleseismic tomography and the mantle xenoliths within other western USA mafic ultrapotassic igneous suites shows that the total lithospheric thickness beneath NW Colorado was probably ~150−200 km at 33 Ma, when the Middle Park lamproite was emplaced. This is an important constraint on tectonomagmatic models for the Cenozoic evolution of this northernmost part of the Rio Grande rift system.

Early Miocene continental extension-related basaltic magmatism at Walton Peak, northwest Colorado: further evidence on continental basalt genesis

The Walton Peak lavas erupted directly onto c. 1.8 Ga basement and were interbedded with subaerial sediments. Four new K-Ar dates for the lavas average 22.8 ± 0.3 Ma, associating them unambiguously with the earliest large-scale extension-related magmatism in NW Colorado. The lavas are basalts, trachybasalts and shoshonites. There is also one 100 m thick composite flow of trachydacite containing pillow-like basic masses (up to tens of metres in size) with chilled margins. Elemental data and Sr and Nd isotopic ratios suggest that the trachydacite evolved from the basalt type forming most of the pillows, by a combination of fractional crystallization and crustal assimilation. Nevertheless, post-evolution magma mixing, during and immediately before extrusion of the flow, has obscured the details of this process. A proportion of the pillow-like basic masses in the composite flow are shoshonites that did not take part in the magma mixing. They are relatively rich in incompatible minor and trace elements; e.g. Nb = 41–46 ppm in the shoshonites and 20–25 ppm in all other Walton Peak basic lavas. The Sr and Nd isotopic ratios of the Nb-rich and Nb-poor compositions overlap. Open-system processes in a long-lived pre-existing magma chamber are considered unlikely to be the main cause of the diverse basic magmas because the Walton Peak lavas directly overlie Precambrian basement. Likewise, crustal contamination models using either upper or lower crustal rock types of this region do not give satisfactory mechanisms to relate the Nb-rich and Nb-poor mafic liquids. A genetic model invoking variable degrees of partial melting of lithospheric mantle containing hydrous minerals does not explain why the Nb-rich and Nb-poor compositions are bimodal and not a range. A variety of sparse ultrapotassic magmas (minettes to lamproites) accompanied the basalt-dominated Neogene volcanism of NW Colorado and surrounding area. Their elemental and isotopic compositions support the view that the ultrapotassic liquids originated by fusion of hydrous and halogen-rich metasomatized zones within the subcontinental lithospheric mantle. Addition of about 15% of lamproitic melt to the low-Nb Walton Peak basic magmas reproduces the main geochemical and isotopic characteristics of the high-Nb magmas. Such a process seems to have been widespread throughout NW Colorado Neogene igneous activity. Although wholly lithospheric mantle sources for all the mafic magmas cannot be ruled out, there is evidence that both subduction-related calcalkaline and ocean-island basanitic melts of asthenospheric origin formed the dominant components of the NW Colorado volcanics. Addition of as little as 10% of lamproitic melt, during their uprise through the subcontinental lithospheric mantle, was sufficient to imprint on them ‘continental’ incompatible element and radiogenic isotope characteristics.