G. L. Hendry

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.

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.

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.

The Flat Tops Volcanic Field: 1. Lower Miocene open‐system, multisource magmatism at Flander, Trappers Lake

The lower Miocene lavas (23–20 Ma) of the Flat Tops volcanic field, NW Colorado, are one of the earliest and most voluminous phases of magmatic activity associated with the development of the Rio Grande rift. Flow-by-flow collections of the lavas at Flander, Trappers Lake, provide evidence for complex open-system magma chamber processes. Geochemically, the lavas range from basalts to shoshonites and have concentrations of major and compatible trace elements that are buffered at higher levels than expected for simple fractional crystallisation, cyclic repetitions (at least five) in the geochemical stratigraphy of the Flander lavas suggest that they are the result of replenishment fractional crystallization (RFC). Variations in some incompatible trace element ratios, such as Ta/Yb and LaN /YbN, and radiogenic isotopes (87Sr/86Sr= 0.70458–0.70607 143Nd/ 144Nd= 0.51226–0.51241) suggest that the lavas have also been contaminated by approximately 10% of Proterozoic 87Sr-rich upper-crust. In addition, anomalously K-rich lava flows with low 87Sr/86Sr occur close to the base and at the top of the succession. It is argued that these are the products of mixing between magmas within the Flat Tops reservoir system and an influx of strongly potassic melt from lithospheric mantle; without subsequent geochemical overprint by the effects of crustal contamination and RFC. All of the lavas have incompatible trace element ratios that mostly resemble those of calc-alkaline basalts; e.g., their chondrite-normalized patterns have troughs at Nb and Ta. These features could result from several alternative processes, such as melting within convecting mantle above a low-angle subducted slab; reaction between various asthenospheric melts and overlying lithospheric mantle; and fusion of metasomatized lithospheric mantle as a result of decompression during regional extension.