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Mafic Underplating in Compressive Volcanic Arc Segments and Cyclic Ramping of Volatile Concentrations in Long-lived (3-8 Myr) Magmatic Reservoirs : Drivers of Metallogenic Fertility

Bruce Rohrlack, Robert Loucks and Michael Palin


Magmatic volatiles such as water, sulphur and chlorine are essential components of ore-forming fluids in porphyry copper and high-sulphidation epithermal copper-gold deposits in volcanic arcs. A spatial and temporal association of magmatic-hydrothermal Cu-Au deposits and compressive stress is evident in many subduction-related volcanic arcs. The clustering of ore-forming episodes in time and space is a broader expression of the effect of tectonic stress on depth of magma chamber entrapment and depth-related chemical trends of magmatic differentiation. We use tectonic/palinspastic reconstructions to constrain the average rate of crustal shortening (~ 18-19 mm year) during the past 7 Myr in southern Mindanao, in the vicinity of the Tampakan stratovolcano complex. The high Al content of hornblende phenocrysts in the Tampakan dacites constrains most of the magmatic differentiation to occur at ~ 5-6 kbars and 18.6-22.3 km depth in a magma chamber near the base of the crust. Volcanic-hosted metamorphic xenoliths from monogenetic volcanoes which have sampled thermally undisturbed country-rock from the base of the crust in other arcs, indicate that average temperatures at the base of the arc crust are typically 750 ± 75°C (hotter than the wet granite solidus). Hence magma chambers trapped in the lower crustal regime tend to cool slowly and to last long enough to undergo multiple replenishments of mantle-derived magma during the course of cooling, differentiation by fractional crystallisation, and intermittent tapping of residual melts from the top of the chamber to feed shallower chambers or volcanic eruptions.

We have used zircon geochronology to parameterise time series in chemical compositions of volcanic whole-rocks and of phenocrysts. The latter are, in turn, used to obtain time series in calculated temperature, oxygen fugacity, and wt% H2O dissolved in the magma, as described by Rohrlach and Loucks (RSES Annual Report, 2000). The time series reveal several major cycles of magma replenishment and differentiation over a period of ~ 8 Myr, as shown in the accompanying figure. This reservoir longevity and continuity in magmatic evolution over several million years, afforded by the hot, deep environment of entrapment, facilitates cyclic build-up of incompatible magmatic volatile components in residual melt fractions over time intervals which are an order of magnitude longer than those in shallow upper-crustal sub-volcanic magma chambers.

Ratios of incompatible on compatible elements in detrital zircons of Late Miocene to Recent age on the flanks of the Tampakan stratovolcano reveal cyclical increases in incompatible element concentrations within the melts from which the zircons crystallised (Figure 4b). The cyclic advancement and retreat of these element ratios is superimposed on a long-term trend to highly evolved incompatible-element-rich compositions. During this 8 million year period, erupted magmas progressively became more fractionated, as defined by increasing silica content of the melts (Figure 4a). They became increasingly water-rich as melts progressed from ~ 2.9 wt/% H2O to ~ 7.3 wt/% H2O (Figure 4c), and became progressively more chlorine-rich as defined by chlorine contents recorded in apatite phenocrysts. Modelling of major-, trace- and rare-earth-element trends in Late Miocene to Recent igneous rock samples from the Tampakan district reveal several magmatic cycles wherein hornblende became increasingly dominant within each fractionating sequence, with hornblende supplanting pyroxene in the crystallisation sequence at progressively earlier intervals in successive magmatic cycles. This Late Miocene to Recent period of increasing volatile contents in the long-lived, Moho-level magma reservoir, sampled by successive stratovolcanoes via a series of small and transient upper crustal chambers, is coeval with a 7 million year episode in which the amount of plate convergence in excess of that not accommodated by subduction was partitioned into compressive intra-crustal folding, thrusting and thickening (Figure 4d).

These geochemical data are interpreted to reflect a long-term build-up of magmatic volatile components within a sub-crustal magma chamber over several million years. Multiple replenishments of primitive magma from the mantle added successive aliquots of volatile components such as water, sulphur and chlorine, to a long-lived magmatic reservoir in which the horizontal compressive stress in the lower crust inhibits escape of buoyant volatile-enriched residual melt fractions by sub-vertical dyke propagation from the deep reservoir. In that case, the residual melt fraction from a cycle of magmatic differentiation can pass on its accumulation of incompatible components (H2O, Cl, SO3) to the next generation of replenishing mantle-derived magma. Through a succession of such differentiation and replenishment cycles, the residual melts may eventually ramp up to exceptional concentrations of incompatibles. High metallogenic fertility is strongly facilitated by high concentrations in magmatic chlorine and water. Because H2O solubility in silicate melts is chiefly a function of pressure, the more H2O-rich a magma is, the deeper it starts exsolving a magmatic hydrothermal fluid. Aqueous fluids exsolved deeper are denser and have much higher partition coefficients of metal-chloride salts into the exsolving fluid, so the metal scavenging efficiency of fluids from parental melt is expected to increase with depth of exsolution.

 

Figure 4: (a) Time series evolution of SiO2 in igneous rocks from the Tampakan stratovolcano complex. SiO2 determined by whole-rock XRF analyses. The samples were dated by 238U-206Pb using LA-ICPMS on zircons and by 40Ar-39Ar dating on hornblende and biotite. (b) Secular evolution of U/Ti measured in detrital zircons from the late Miocene to present using laser-ablation ICP-MS. (c) Magmatic water contents determined using the Housh and Luhr (1991) plagioclase-silicate melt "geohygrometer", and which was subsequently refined by Rohrlach and Loucks (2000) to account for the effect of dissolved H2O in depressing the melt's silica activity. (d) Estimations of the components of the total convergence of the Philippine Sea Plate (PSP) with respect to the Sunda Block (continental SE Asia), parallel to the PSP convergence vector (288°N), which has been partitioned between various "convergence accommodating structures" within the southern Mindanao block since the Late Miocene. The patterned areas represent relative proportions of convergence which is accommodated at the Sulu, Halmahera, Sangihe, Cotabato and Philippine subduction zones. The shaded pink area represents the residual component of convergence which is available for intra-plate deformation and thickening in southern Mindanao. The tie-points for the diagram were constrained at 0 Ma (A, Su, C, P) by modern GPS data (Rangin 1999), and in the past by long term average subduction rates determined from seismically identifiable slab lengths within the mantle (S, S1, H, H1), and from tectonic models for the age of subduction zone initiation and termination (Po, Co, S2, H2). Major recharge events from the mantle into the deep or sub-crustal magma chamber coincide with the troughs in Figures 4a-c. The period of crustal compression (7 Ma to 0 Ma; Figure 4d) is comparable with the time-scale of differentiation within the Tampakan district deep magmatic system. Crustal stress release at ~ 3.8 Ma (base of the orange-shaded trough) occurs just after initiation of the Philippine trench at 4 Ma, and may be responsible for the major recharge event between 3.4 Ma and 3.8 Ma as distributed tectonic stresses in the lithospheric mantle are relaxed by initiation of the Philippine subduction system.