The Growth and Evolution of Large Silicic Magma Bodies II
Presiding: O Bachmann, University of Geneva; C F Miller, Vanderbilt University
V21A-01 08:30h
Multiple Magmatic Lineages and the Source of K in High-K Calcalkaline Granitoid Suites: the Arrochar and Garabal Hill-Glen Fyne Intrusions, Scotland
The spatially associated, High-K, I-type Garabal Hill, Glen Fyne and Arrochar intrusions (ca 425 Ma) intrude Dalradian metasediments. Major rock types are porphyritic gabbro and K-feldspar-phyric granite. Minor rock types include ultramafic rocks, diorite, quartz monzodiorite, granodiorite, pegmatite, leucogranite and granophyre. The granitoids contain igneous, microgranular enclaves (typically rounded and < 10 cm across) that are more mafic than their hosts. Geochemical data, portrayed on Harker plots, suggest that all these rocks could be related by some fairly simple processes such as crystal fractionation and/or magma mingling and mixing. In such models, the mafic to intermediate enclaves are commonly interpreted as intermediates in the mixing process - hybridised remnants of the mafic end member. Here we pose the questions: are these simple models viable and what end members may have been involved? Backed up by detailed petrographic study, we have carried out geochemical and isotope (Rb-Sr and Sm-Nd) analyses of a large number of rocks from the area. These data show that, far from being simple, the magmas were derived from several different mantle source materials, and at least two crustal reservoirs. We show that the enclave suite of the Glen Fyne granite does not form part of the host magmatic lineage. The enclaves represent a group of accessory hybrid magmas produced by mixing between a mantle-derived mafic end member and a Glen Fyne granitic magma. The main trends for the Arrochar and Glen Fyne intrusions are also characterised by mixing between mafic and more felsic end members. Contrary to what might be expected in high-K calcalkaline series, the high K content in the magmas is derived from the mantle end member rather than directly from the crust. This could be a general feature in such suites. Finally, there is significant isotopic variation among the felsic Glen Fyne rocks. A good deal of the geochemical variation in all these magmas was either inherited directly from the protoliths (in the mantle and crust) or was produced by magma mixing. There is little evidence for crystal fractionation, though it locally produced some of the most felsic magmas.
V21A-02 INVITED 08:45h
Zircon Saturation and the Viability of Magma Bodies During Intrusion of the Tuolumne Intrusive Series, Sierra Nevada Batholith, California
The Tuolumne Intrusive Series (TIS) is a prime example of a zoned arc intrusion, and is one of a belt of large (>1000 km2) consanguineous, Cretaceous, zoned intrusions within the Sierra Nevada batholith. Past work linked the differentiation of the TIS with field petrology and modification of magma in situ in large reservoirs. New single-zircon geochronology and field studies show that the TIS was assembled from numerous magma inputs from 94-85 Ma, casting doubt on the existence of a large body of mobile magma during growth and emplacement [1], and calling into question where the bulk of the chemical differentiation occurred (exposure level or deep source). Zircon saturation temperatures (Tzrc; [2]) for the TIS are useful for understanding its assembly, particularly in combination with high precision, single zircon ages. The two largest units of the TIS (Half Dome and Cathedral Peak granodiorites) have low calculated Tzrc (Half Dome: 710-765°C, mean=740°C; Cathedral Peak: 715-780°C, mean=760°C), which implies complexities in how zircon ages are used to interpret the construction of the TIS: (1) at T's appropriate for anatexis, transport, and initial emplacement of granitoid magmas, source-derived zircons would be dissolved; zircons in TIS thus crystallized during post-emplacement conductive cooling upon reaching Tzrc, or were inherited late; (2) because Tzrc is not far above the solidus, appreciable age differences (several 105 yr?) might be produced by slow cooling of a large magma body that was open to heat input; heat added need only have been sufficient to maintain near-eutectic conditions, perhaps as persistent mush, but with net cooling (qout > qin), such that ages track the migration of the Tzrc isotherm during solidification; (3) reheating of any mush, or melting of intrusions associated with earlier inputs to T's >> Tzrc would dissolve older zircons on time scales of 104-105 years (e.g. [3]); this time scale sets a limit on the longevity of melt-rich, magma bodies formed by remelting of earlier intrusions at a scale that encompasses samples with statistically distinguishable zircon ages. A corollary of 1-3 above is that there may be a correlation between zircon age dispersion and inferred magma body longevity in the TIS. Samples with tight clusters of concordant, single-crystal weighted mean ages (low MSWD) presumably correspond to areas in TIS where no appreciable heating (several 100's °C) occurred after cooling through Tzrc. Conversely, samples showing concordant, single-crystal, age dispersion may reflect: (a) reheating/melting of mushy zones or earlier solidified intrusions within the TIS, such that older zircons were incorporated into new inputs of magma but did not completely dissolve, and/or (b) transient thermal oscillations about Tzrc in long-lived mushy magma, producing periods of alternating dissolution and growth. Age progression from older to younger toward the interior would still be preserved even with these complexities. [1] Coleman DS, Gray W, Glazner AF (2004) Geology 32:433-436; [2] Watson EB, Harrison TM (1983) Earth Planet Sci Lett 64:295-304; [3] Watson EB (1996) Trans Roy Soc Edinburgh: Earth Sci 87:43-56.
V21A-03 09:00h
The Spirit Mountain Batholith: Documenting Magma Storage in the Upper Crust One Pulse at a Time
More than 500km2 of Miocene intrusive rocks in the Newberry Mountains, southernmost Nevada, comprise a batholith that has been tilted west up to 50° (Hopson et al, 1994) and exposed due to late Miocene extension and crustal thinning in the Colorado River Extensional Corridor. New zircon U/Pb data (SHRIMP) demonstrate episodic crystallization of the batholith during four main episodes of magmatism that are consistent with field evidence: 17.2 Ma, 16.4-16.0 Ma, 15.8 Ma, and 15.6 Ma (errors are ±0.2 [2σ] with MSWD <2). Formerly referred to separately as the Spirit Mountain Pluton (SMP) and Mirage Pluton (Howard et al., 1996; Hopson et al, 1994), new geochronological, geochemical, and field evidence suggest that the Mirage is closely related and that the SMP itself is composite. Thus, we suggest these intrusions should be considered collectively as the Spirit Mountain Batholith. Rocks emplaced at 17.2 Ma include high-silica equigranular granites and porphyries (~77% SiO2) at the shallowest levels of the batholith. The 16.4 - 16.0 Ma unit is the largest, consisting of multiple sequences of coarse to medium grained granite grading to magmatically foliated quartz monzonite with prominent accumulated feldspar crystals. These sequences range from ~60-73 % SiO2, and appear to represent discrete intrusive episodes. Local inter-pluton cupolas of high-silica granite (~77% SiO2) document fractionation and melt segregation within individual pulses. Minor mafic input into the pluton during this period is manifested by magmatic enclaves ranging from mm's to 2 m in size, locally abundant and highly flattened into pancake shaped discs parallel to the foliation of the host. The 15.8 Ma episode consist primarily of fine grained biotite granite (~73% SiO2) that forms irregular, initially subhorizontal sheets (m's to <100 m thick). A large sheet of gabbro appears to have mingled and locally mixed with the biotite granite and yields an identical age. The 15.6 Ma pulse is a series of N-striking felsic (~73% SiO2) to mafic dikes that cuts all other units (George et al, 2005). Field relations suggest that the two latest episodes of magmatism (15.8 Ma, 15.6 Ma) are associated with the Mirage Pluton. Xenolith relations along with sharp contacts between the granites show that these later magmas intruded the SMP after it had become rigid. Though many different pulses of magma are involved in this system, we contend that given their proximity in space, time, and composition, this series of intrusions constitutes a single composite batholith that formed in response to continuing input of very similar magma into the upper crust. Intrusion styles were likely dictated by the changes in regional tectonics. For example, at the onset of extension, diking may have ensued, as opposed to magma ponding in thick, slowly solidifying sheets. Because of tilting, this field area serves as a cross section, and thus a window into the processes of batholith construction, including magma injection, magma interaction, differentiation of individual magma pulses, and compaction of crystal mushes.
V21A-04 INVITED 09:15h
Constraints on the source and evolution of silicic magmas from diverse tectonic environments
Calc-alkaline silicic magmas (>65 wt. % SiO2) occur in a wide range of tectonic environments and diverse models have been proposed for their origin. They are common in continental convergent zones and most models for the origin of silicic magmas in these areas involve partial melting of continental crustal rocks. In these models the compositions of the silicic magmas vary according to the relative contribution of evolved continental crust and mantle-derived melts. In contrast, silicic magmatism has been considered to be minor in intra-oceanic arcs, where evolved crust is absent. However recent studies have shown that silicic magmatism can be a significant component in intra-oceanic arcs. These studies propose that the generation of silicic magmas in ocean arc systems involve the partial melting of recently emplaced, mantle derived, stalled (crystallized) calc-alkaline magmas. Abundant silicic magmas can be produced both with and without the presence of evolved continental crust. This paper evaluates the source and evolution silicic ignimbrites and related deposits in three areas: 1.)Southwest Nevada, USA, which is associated with rifting; 2.)In Central America associated with the active volcanic front; 3.)In the Macolod Corridor, Luzon, Philippines, which is associated with subduction and rifting. Fractional crystallization from primitive basaltic or primitive andesitic magma can be rejected as the process that produced the silicic magmas in these areas. This conclusion is based on oxygen isotopes, Sr and Nd isotopes, and very high K2O/Na2O values in the silicic magmas. In the southwest Nevada individual, large-volume, ash-flow sheets contain two or more silicic magma batches along with a more mafic magma batch, all of which are independently generated. In Central America, the along arc variation of the silicic deposits with respect to selected trace element ratios (Ba/La, U/Th, Ce/Pb), oxygen isotopes, Nd and Sr ratios mimic the along arc variation in the basaltic lavas from the active arc. Some ignimbrite units have as many as seven distinct and independent magma types. In the Macolod Corridor, Philippines, at least three distinct silicic compositions are recognized. A common factor in all of these silicic deposits is their high K2O/Na2O values, which cannot be produced by fractional crystallization of primitive magmas. Data from melting experiments of primitive basalts and andesites demonstrate that it is impossible to produce high K2O/Na2O silicic magmas by fractional crystallization or partial melting of a low- K2O/Na2O source. In areas with evolved continental crust, such as southwest Nevada, this is not a problem. However, in oceanic arcs an enriched K2O/Na2O source is required. Partial melting or extreme fractionation of evolved K-rich basalts can produce the high K2O/Na2O silicic magmas observed. In Central America and the Macolod Corridor, our data are consistent with the model that large-volume silicic magmas result from melting of ponded K-rich basaltic plutons, or melt extraction from these partially crystallized plutons. In the case of southwest Nevada the diverse silicic magmas are produced by melting of different crustal sources with little mixing with a mantle-derived source. Extension may be a fundamental control for producing large-volume silicic ignimbrites because space for large magma bodies is produced in releasing bends or pull-apart structures.
V21A-05 09:30h
Contrasting Modes for Granitic Batholith Construction: the Role of Tectonic Stress
Mental images of felsic magma chambers are inextricably linked to eruption of voluminous high silica ignimbrites which require the presence of even larger volume magma chambers within the crust. Systematic stratigraphic compositional variation within ignimbrites indicate compositionally stratified chambers, with a high silica cap, grading downward to intermediate compositions, all underlain by mafic magma. Influx of basaltic magma keeps the overlying felsic magma liquid and convecting, thus sustaining the large volume chamber. Too large a mafic replenishment can catastrophically extinguish the chamber by triggering an eruption. Silurian granitic batholiths of the coastal Maine magmatic province fit well with an origin as the plutonic roots for such volcanic systems e.g., Mount Desert Igneous Complex (Seaman, 1999), Vinalhaven Igneous Complex (Hawkins and Wiebe, 2004). Large volcanic eruptions are associated with these batholiths. In contrast to "open system" magma chambers, isotopic evidence indicates large "closed system" granitic batholiths can be assembled incrementally from coalescence of discrete smaller batches of felsic magma. Observed heterogeneity in initial isotopic compositions (e.g., Sr, Pb) from these batholiths is interpreted to be inherited form the source region. Preservation of this isotopic heterogeneity eliminates homogenization by convective mixing, indicating discrete batches of magma welded together to form large granitic batholiths. The Devonian Lucerne granite of the coastal Maine magmatic province fits this style of "closed system" batholith construction (Hogan and Sinha, 1991). It is indeterminate as to whether or not the Lucerne magma chamber erupted. The distinction between these two styles of batholith construction, and their magma chambers dynamics, is dependent upon the availability and volume of basaltic magma within the crustal column, which is linked to tectonic stress (Hogan et al., 1998). Increased magma driving pressures during periods of crustal extension enable higher density basaltic liquids to rise further in the crustal column, increasing the probability of intersection and eventual trapping within shallow felsic magma chambers, and the formation of "open system" granitic batholiths. Lower magma driving pressures during periods of relaxed extensional stress or compression promotes intraplating of mafic magma in the deep crust, leading to large scale crustal anatexis, the rise of buoyant felsic magmas, and the formation of "closed system" granitic batholiths.
V21A-06 INVITED 09:45h
Hydraulic Inflation and Buoyancy Pumping: A Model for Large, Fracture-Mediated Felsic Intrusions
Large, intraplate, felsic intrusions with crustal anatectic sources, pipe-like conduits and tabular plutons are analysed, emphasising the magma-intrinsic factors that control their development. Conductively heated magma sources are broad and domical. Volume changes during melting generate buoyancy and hydraulic magmatic loads. The deformation state of the lithosphere determines its response. Rock strength depends on loading rate and limits differential stress. Tensile failure requires magma pore pressure, to reduce confining stress, while magma wedging in cracks modifies the stress field for vertical cracking. Non-magmatic loads include gravity and, critically, horizontal tensile stress from uplift, which favours vertical cracks. Tectonic forces are secondary. Intrusion begins with instability, due to the presence of magma and feedback between magmatic and ambient source loading. Hydrostatic magma pressure PM is augmented by buoyancy and melting dilativity overpressures (ΔPB and ΔPV). ΔPBmax increases with source height h while non-relaxed dilation creates ΔPV (proportional to h3), which arises, instantaneously, to the wall-rock strength. ΔPV confers high mechanical efficiency and increases faster than ΔPB to a max. that is orders of magnitude greater. Inelastic uplift results but, since full inelastic relaxation of ΔPV is impossible, elastic source compression results, providing hydraulic impetus for brittle source rupture and magma flow. For extensive melting, a molten cavity may develop. At lower melt fractions, melt veins form, connect and propagate as dykes. Magma pore overpressure and wedging fulfil the stress criteria for tensile rupture, whose geometry depends on the initial stress field (σH- or σV-dominant). In domed crust, radial ruptures focus to form a wide, central, pipe-like conduit. Requirements for a viable conduit are; (i) σV1; (ii) a positive gradient in PM, and; (iii) an aperture adequate to prevent magma freezing. If PM in dykes increases σH to σH1, then dykes re-orientate to sills, terminating ascent and initiating emplacement. Once stagnated, static ΔPB is typically sufficient to force sill injection at depths less than a critical value D, where ΔPB = σV. Sills growth is dominated by floor depression. Underburden subsidence suppresses roof uplift, influences the sill's plan geometry, expels source magma, processes crust through the melting zone, decreases σH in down-warped crust to favour conduit widening and magma ascent, and drains the source. Hydraulic inflation may end by melting cessation or exhaustion of excess magma volume EMV. Inelastic source swelling (φ) or crack growth (η) relax EMV. Exhaustion of non-relaxed EMV (EMV*) divides intrusion into two regimes; hydraulic inflation and buoyancy pumping. Loss of hydraulic drive (EMV* = 0) occurs either during dyke or sill growth, depending on the initial EMV (source volume and melting dilativity). Once rupture occurs, disequilibrium cracking initiates, and η increases abruptly as stored EMV* converts to crack volume. Equilibrium cracking begins once reduced EMV* balances new crack growth. If a sill is available to decouple shallow and deep crust, buoyancy pumping initiates, where underburden subsidence empties the magma source.