Showing papers in "Journal of Petrology in 1988"

The Generation of Granitic Magmas by Intrusion of Basalt into Continental Crust

Abstract: When basalt magmas are emplaced into continental crust, melting and generation of silicic magma can be expected. The fluid dynamical and heat transfer processes at the roof of a basaltic sill in which the wall rock melts are investigated theoretically and also experimentally using waxes and aqueous solutions. At the roof, the low density melt forms a stable melt layer with negligible mixing with the underlying hot liquid. A quantitative theory for the roof melting case has been developed. When applied to basalt sills in hot crust, the theory predicts that basalt sills of thicknesses from 10 to 1500 m require only 1 to 270 y to solidify and would form voluminous overlying layers of convecting silicic magma. For example, for a 500 m sill with a crustal melting temperature of 850 °C, the thickness of the silicic magma layer generated ranges from 300 to 1000 m for country rock temperatures from 500 to 850 °C. The temperatures of the crustal melt layers at the time that the basalt solidifies are high (900-950 °C) so that the process can produce magmas representing large degrees of partial fusion of the crust. Melting occurs in the solid roof and the adjacent thermal boundary layer, while at the same time there is crystallization in the convecting interior. Thus the magmas formed can be highly porphyritic. Our calculations also indicate that such magmas can contain significant proportions of restite crystals. Much of the refractory components of the crust are dissolved and then re-precipitated to form genuine igneous phenocrysts. Normally zoned plagioclase feldspar phenocrysts with discrete calcic cores are commonly observed in many granitoids and silicic volcanic rocks. Such patterns would be expected in crustal melting, where simultaneous crystallization is an inevitable consequence of the fluid dynamics. The time-scales for melting and crystallization in basalt-induced crustal melting (10—10 y) are very short compared to the lifetimes of large silicic magma systems (>10 y) or to the timescale for thermal relaxation of the continental crust (> 10 y). Several of the features of silicic igneous systems can be explained without requiring large, high-level, long-lived magma chambers. Cycles of mafic to increasingly large volumes of silicic magma with time are commonly observed in many systems. These can be interpreted as progressive heating of the crust until the source region is partially molten and basalt can no longer penetrate. Every input of basalt triggers rapid formation of silicic magma in the source region. This magma will freeze again in time-scales of order 10—10 y unless it ascends to higher levels. Crystallization can occur in the source region during melting, and eruption of porphyritic magmas does not require a shallow magma chamber, although such chambers may develop as magma is intruded into high levels in the crust. For typical compositions of upper crustal rocks, the model predicts that dacitic volcanic rocks and granodiorite/tonalite plutons would be the dominant rock types and that these would ascend-from the source region and form magmas ranging from those with high temperature and low crystal content to those with high crystal content and a significant proportion of restite. I N T R O D U C T I O N One of the central questions in igneous petrology concerns the generation of silicic magmas. There is now convincing evidence that most of the large plutonic complexes of granite in the continental crust are the result of crustal anatexis (Pitcher, 1987). There is also [Journal of Petrologf, Vol. 29, Ptn 3, pp 599-«24, 1988] © Oxford Umvcroty Prcu 19S8 600 HERBERT E. HUPPERT AND R. STEPHEN J. SPARKS widespread evidence that basaltic magma from the mantle is often intimately associated with the generation of silicic magmas (Hildreth, 1981). This association of mafic and silicic magmas can occur in orogenic belts above subduction zones, in continental hot-spots, and in regions of crustal extension. In plutonic complexes, mafic and intermediate igneous activity are recorded in contemporaneous dyke swarms, small satellite intrusions, and in mafic enclaves within the granites (Vernon, 1983; Pitcher, 1986, 1987). In silicic volcanic centres, evidence of basaltic magmatism is found in satellite lava fields and cinder cones, early lava shields and stratovolcano complexes prior to the main silicic volcanism (Lipman, 1984), and as mafic inclusions and bands within the silicic volcanic rocks (Smith, 1979; Bacon, 1986). Petrological and geochemical features of many silicic igneous rocks are also convincingly explained by admixture of a mantle-derived (mafic) component with a crustal melt. Regions of high temperature and low pressure metamorphism are commonly associated with granite plutonism and a plausible explanation of this association is that basalt is intruded into the crust, causing melting and high heat flow. Indeed basalt underplating of the crust is a currently popular idea to explain both large scale crustal melting and the strongly layered character of the lower crust. While there may be some silicic magmas that are generated by processes without the aid of basaltic input, such as tectonic thickening of radioactive crust (England & Thompson, 1984; Pitcher, 1987), this paper takes the position that in many cases the additional thermal energy of basalt is essential. The continental crust is strongly layered in terms of its composition, density, and mechanical behaviour. The upper crust is cold and brittle whereas the lower crust is hotter, has a higher density, deforms in a ductile manner, and is commonly characterized by prominent horizontal layering. Basalt magma can be emplaced into the continental crust as dykes and sills and, in some cases where the rate of magma input is high, these intrusions can coalesce to form larger magma chambers. Dyke emplacement does not seem an efficient way of generating large volumes of silicic magma, because dykes are usually small in width and much of the potential heat for melting will not be utilized if the mafic magma erupts. Sills provide a more promising situation in which extensive crustal melting can occur. Horizontal intrusions concentrate their heat at a particular level in the crust and do not dissipate their heat over a large depth range. Sills are intrinsically more efficient than dykes in this respect. Dykes may play an important role in heating up the crust to initiate melting. However, once a region of the crust has become hot, ductile, and partially molten, conditions for dyke propagation become less favourable. A layer or region of partially molten crust provides an effective density barrier and we suggest that basalt magma reaching such a level will spread out as horizontal intrusions. An additional factor which promotes sill formation in the lower parts of the crust is its strongly layered character providing a structural environment in which horizontal intrusions are favoured. For these reasons this paper is concerned principally with the heat transfer and fluid dynamics of sills intruded into hot continental crust. We consider the cooling and crystallization of basaltic sills emplaced into the continental crust. In particular, we emphasize the situation where the roof of the sill is composed of rock which has a fusion temperature that is lower than the magma temperature and the roof rock consequently melts. This is likely to be the normal situation where basalt intrudes into the typical rock types of mature and ancient middle and upper crust which are already at high temperature. However, the concepts developed in this paper are also likely to be applicable to conditions in immature continental crust such as in island arcs, to more refractory lower crust and to lower crust formed by slightly older or even contemporaneous episodes of basalt underplating. In each of these latter cases, lithologies which have relatively low fusion temperatures can form by differentiation processes and can be remelted by further intrusion THE GENERATION OF GRANITIC MAGMAS 601 of basalt. Thus the model is not confined to the origin of granites, but should be relevant to the origins of intermediate rocks such as tonalites and evolved alkaline rocks such as syenite. We present experimental studies on the melting of the roof of a sill. We develop a quantitative model of the melting process at the roof, which describes the rates at which a new layer of roof melt forms and the rates at which the underlying liquid layer solidifies. We discuss possible mechanisms by which the melts can be mixed together and also their implications for magma genesis within the continental crust. A companion paper (Huppert & Sparks, 1988a) describes the melting of the roof of a chamber from a detailed fluid mechanical point of view. Throughout this paper the magma will be considered to be Newtonian. Although magma in reality can be non-Newtonian, especially when it is rich in crystals (McBirney & Murase, 1984) its nonlinear Theological properties and the consequences of its non-Newtonian rheology are poorly understood. Two effects may be evident: there may exist a yield strength, so that for a sufficiently low applied stress the magma will not move; and the nonlinear viscosity may alter the heat flux transferred by a convecting magma. Because of the relatively large values of the Rayleigh number that result in most of our calculations, we anticipate that the yield strength will be exceeded by quite a margin. The alterations in the heat flux are at the moment difficult to anticipate and we suggest that the reader views our quantitative results as an indication of the calculated quantity rather than as a precise value. It may be possible to examine non-Newtonian effects with greater insight in the future, but a Newtonian description illuminates many of the fundamental effects and is a necessary first step in order to form the basis for any comparison. EXPERIMENTAL STUDIES The geological problem i

Anhydrous Partial Melting of a Fertile and Depleted Peridotite from 2 to 30 kb and Application to Basalt Petrogenesis

Abstract: Reversal experiments have been performed to check the method of Jaques & Green (1980) to determine equilibrium partial melts from peridotite compositions. Reversals of the Jaques & Green (1980) calculated equilibrium partial melt (CEPM) compositions have been carried out in two ways: (1) By running CEPM compositions at original P and T conditions and testing for multiple saturation in residual phases of the original experiments. (2) By sandwich/mixed experiments using the CEPM composition plus peridotite (either Hawaiian pyrolite or Tinaquillo lherzolite). The glass (liquid) compositions from the first series of experiments show that the CEPM compositions of Jaques & Green (1980) are too olivine-rich. The glass (liquid) compositions from the second series of experiments define new olivine+orthopyroxene�clinopyroxene cotectics in a molecular normative tetrahedron. The new cotectics plot towards the Qz apex of the tetrahedron, away from the cotectics defined by the CEPM compositions of Jaques & Green (1980). Partial melt compositions have also been determined at 20 and 30 kb, using both the sandwich technique and the approach by modal analysis and mass balance. The results of the experimental study are used to evaluate the petrogenesis of mid-ocean ridge basalts, Hawaiian tholentes and primary magmas in intraoceanic convergent margin settings.

Anhydrous Partial Melting of Peridotite from 8 to 35 kb and the Petrogenesis of MORB

Abstract: Equilibrium partial melts under anhydrous conditions have been determined for a MORB pyrolite composition (considered suitable for the production of primary MORB melts) from 8 to 35 kb and a relatively more depleted peridotite composition, Tinaquillo lherzolite, at 15 and 20 kb. The equilibrium partial melts were determined using ‘sandwich’ experiments. The primitive MORB glass DSDP3-18-7-1 was used in experiments using MORB pyrolite while a calculated liquid composition from Jacques & Green (1980) was used in experiments with Tinaquillo lherzolite. The equilibrium partial melts in conjunction with previous studies are used to establish a melting grid from 5 to 35 kb in the CIPW normative basalt tetrahedron. Equilibrium partial melts from MORB pyrolite combined with constraints imposed by the composition of primitive MORB glasses, MORB picrites and primitive trapped glass inclusions are subjected to olivine fractionation calculations to establish the range in composition of primary MORB melts. The results suggest primary MORB melts segregate from MORB source diapirs at pressures of 8 to 25kb in equilibrium with either lherzolite or harzburgite residues. MgO contents of primary MORB melts range from 10–17 wt.% while primary melts with > 17 wt.% MgO are of minor importance. Primary MORB melts range in composition from slightly ne-normative and more hy-normative picrites to olivine and quartz normative olivine tholeiite. Although some primitive MORB glass compositions are possible primary melts from 8–20 kb the majority of primitive MORB glass compositions are derived by olivine fractionation (11–25 wt.%) from more picritic parents from pressures of 15–25kb. The results of this study are in agreement with previous workers suggesting that there is an array of primary MORB melt compositions which evolve to multiple saturation in olivine+ plagioclase + clinopyroxene at different points along a latm. cotectic via differing degrees of olivine fractionation.

Recent Volcanism in the Puyehue-Cordon Caulle Region, Southern Andes, Chile (40 5 S): Petrogenesis of Evolved Lavas

Abstract: Apparatus for producing stationery assemblies comprising means for moving a plurality of superposed webs along a path of travel, tab-cutter which cuts a plurality of superposed tabs in said webs and bends the tabs so that their free ends extend outwardly of the webs and in a leading direction along said path of travel, and a roller adhesive applicator which rotates in a direction opposite to the direction of movement of the webs. A controlled amount of quickdrying adhesive is simultaneously applied to the free ends of the tabs by the roller, with the position of the roller relative to the webs and the speed of rotation thereof being adjustable to vary the amount of adhesive applied to the free ends of the tabs and also to vary the area of the tabs to which the adhesive is applied.

The Perseverance Ultramafic Complex, Western Australia: The Product of a Komatiite Lava River

Abstract: The Perseverance ultramafic complex is a body of olivine-rich komatiitic rocks spatially associated with the Agnew nickel deposit, in the Agnew-Wiluna greenstone belt of the Archaean Yilgarn Block in Western Australia. The complex consists of a central lenticular body, up to 700 m thick, of olivine adcumulates, flanked by laterally extensive sheet-like bodies of olivine orthocumulates and spinifextextured komatiite flows. Rocks progressively further away from the central lens have chemical compositions reflecting higher original proportions of komatiite liquid to cumulus olivine. Parent liquids had MgO contents between 25 and 32% MgO, approximately chondritic Al/Ti ratios and HREE patterns, and moderate depletion in LREE. Olivines within the adcumulate lens show a progressive increase in forsterite content from Fo93 at the bottom to Fo94..5 at the top. Calculated original olivine compositions in ihe flanking rocks are similar to those at the base of the central lens. Original olivine nickel contents show a symmetrical variation from maximum values of 3500 ppm at the top of the central lens, through minimum values of 1000 ppm at the base and margins of the central lens to intermediate values in the distal rocks. The complex as a whole shows evidence for nickel depletion relative to other komatiite suites. These observations are explained in terms of prolonged eruption and flow of komatiitic lava down a major flow channel or lava river. Adcumulates crystallized on the floor and sides of the central channel, which was formed at an early stage by thermal erosion of floor rocks. Episodic overflow of the central channel produced distal 'flood plain' rocks consisting of olivine orthocumulates and layered flows. Lavas became more magnesian and nickel-rich with time, giving rise to the observed spatial variation in primary olivine composition. Nickel depletion of the earliest lavas is attributed to pre-eruption segregation of large volumes of immiscible Fe-Ni-sulfide, which were concentrated to form the underlying Agnew nickel deposit.

An Overview of the Geochemical and Isotopic Characteristics of the Eastern Australian CainozoicVolcanic Provinces

Abstract: A broad zone of intra-plate volcanism occurs for some 3000 km along eastern Australia. Mafic lavas dominate, and include the following types (with frequency % occurrence, based on 1757 analyses): Leucitites (21), melilitites, nephelinites, and analcimites (5-4), basanites (12-7), alkali basalts (7-0), ne- hawaiites and hawaiites (44-4), transitional basalts and Ol-tholeiites (17-4), and Q-tholeiites (11-0). These lavas are erupted through a wide variety of crustal-tectonic environments, from Proterozoic to Mesozoic. Marked differences in chemistry exist between the lavas erupted from central volcano provinces (in which most ‘evolved’ lava types occur) and lava-field provinces, the former exhibiting greater isotopic variability and evidence for more extensive crystal fractionation (AFC). More evolved lava types include mugearites, benmoreites, icelandites, peralkaline and non-peralkaline trachytes and phonolites, comendites, low-silica and high-silica rhyolites. Marked regional differences exist with respect to distribution of various lava types; northern Queensland and Tasmania, for example, apparently have very few strongly evolved lavas, the latter region also containing a disproportionately high percentage of nephelinites. Trace element geochemistry of the mafic lavas is very variable, but typically continental; the lavas are enriched in incompatible elements, but enrichment varies greatly, being extreme in the leucitites, melilitites, and nephelinites, and slight (relative to MORB) in certain Q-tholeiites. It is shown that the patterns of the more extreme incompatible element enrichments are consistent with recent work on extraction of small melt fractions. Nevertheless, marked source inhomogeneities are indicated by the data, believed to be lithospheric; arc-modified lithosphere is suggested as a source for at least some lava field tholeiites. It is clear, however, that the majority of lavas have been modified by some degree of low-medium pressure crystal fractionation processes (olivine ±augite ± plagioclase ± Fe-Ti oxides). The critical role of fractional crystallization is even more apparent in the chemistry of the intermediate and silicic lavas, which exhibit dual patterns of progressive and ultimately extreme element enrichment (e.g., Pb, Th) and depletion (e.g., Mg, V, Ni, Cr, Sr, Ba, Eu). These patterns are readily modelled by Rayleigh fractionation, but require elevated K values, appropriate to silicic magmas; continually varying Ks are also indicated by some data sets. The mafic lavas exhibit a wide, but continuous variation of isotopic compositions, there being marked regional differences, but with the leucitite exception, no particular compositional ranges characterize particular compositional types. Correlations are observed between Sr, Nd, and to a less extent Pb isotopic compositions with, for example, Bh/Th, Ba/Nb, and mg-ratios. Much of the observed geochemical-isotope data, excepting the most undersaturated lavas, can be modelled in terms of AFC processes, utilizing upper and lower crustal models. The isotopic data of the alkaline Tasmanian lavas are distinctive and are interpreted as asthenospheric; these compositions, and those of rare magnesian alkaline lavas from elsewhere in the region, suggest a mixed mantle source containing a component approaching the ‘St. Helena-type’. The leucitites have a marked DUPAL isotopic signature, and it is noted that these occur above an interpreted Proterozoic rift system, suggesting a lithospheric source. Isotopic and geochemical data for the trachytes and low-silica rhyolites are consistent with AFC processes, with variable assimilation, modelled in terms of upper crustal components. The high-silica rhyolites are isotopically distinctive, and are interpreted as local upper crustal melts, but modified by subsequent crystal-liquid fractionation.