Sulfur and metal fertilization of the lower continental crust

Abstract

Mantle-derived melts and metasomatic fluids are considered to be important in the transport and distribution of trace elements in the subcontinental lithospheric mantle. However, the mechanisms that facilitate sulfur and metal transfer from the upper mantle into the lower continental crust are poorly constrained. This study addresses this knowledge gap by examining a series of sulfide- and hydrous mineral-rich alkaline mafic-ultramafic pipes that intruded the lower continental crust of the Ivrea-Verbano Zone in the Italian Western Alps. The pipes are relatively small (< 300 m diameter) and primarily composed of a matrix of subhedral to anhedral amphibole (pargasite), phlogopite and orthopyroxene that enclose sub-centimeter-sized grains of olivine. The 1 to 5 m wide rim portions of the pipes locally contain significant blebby and disseminated Fe-Ni-Cu-PGE sulfide mineralization. Stratigraphic relationships, mineral chemistry, geochemical modelling and phase equilibria suggest that the pipes represent open-ended conduits within a large magmatic plumbing system. The earliest formed pipe rocks were olivine-rich cumulates that reacted with hydrous melts to produce orthopyroxene, amphibole and phlogopite. Sulfides precipitated as immiscible liquid droplets that were retained within a matrix of silicate crystals and scavenged metals from the percolating hydrous melt, associated with partial melting of a metasomatized continental lithospheric mantle. New high-precision chemical abrasion TIMS U-Pb dating of zircons from one of the pipes indicates that these pipes were emplaced at 249.1 ± 0.2 Ma, following partial melting of lithospheric mantle pods that were metasomatized during the Eo-Variscan oceanic to continental subduction (~420-310 Ma). The thermal energy required to generate partial melting of the metasomatized mantle was most likely derived from crustal extension, lithospheric decompression and subsequent asthenospheric rise during the orogenic collapse of the Variscan belt (< 300 Ma). Unlike previous models, outcomes from this study suggest a significant temporal gap between the occurrence of mantle metasomatism, subsequent partial melting and emplacement of the pipes. We argue that this multi-stage process is a very effective mechanism to fertilize the commonly dry and refractory lower continental crust in metals and volatiles. During the four-dimensional evolution of the thermo-tectonic architecture of any given terrain, metals and volatiles stored in the lower continental crust may become available as sources for subsequent ore-forming processes, thus enhancing the prospectivity of continental block margins for a wide range of mineral systems.

Figure 1 –

Simplified geological map of the Ivrea-Verbano Zone showing the location of the pipes. Modified from Fiorentini and Beresford (2008).

Figure 2 –

Photographs showing outcrops in the Valmaggia mine. (A) Central part of the Valmaggia pipe, near sample site VMG-7; (B) Sharp contact between the Valmaggia pipe and the host gabbro; (C) Sulfide mineralization in the rim portion of the Valmaggia pipe near sample site I-7. A detailed map of the Valmaggia mine has been provided by Fiorentini et al. (2002).

Figure 3 –

Back scattered electron images of samples from the Valmaggia pipe showing (A) the typical pipe silicate assemblage (sample VMG-7), and (B) semi-massive sulfide mineralization (sample I-7). Chr: chromite, cpx: clinopyroxene, ol: olivine, opx: orthopyroxene, parg: pargasite, phl: phlogopite, pn: pentlandite, po: pyrrhotite.

Figure 4 –

Olivine mineral chemistry for each pipe sample illustrated in binary plots of (A) Ni vs. Fo, (B) Mn vs. Fo, (C) Co vs. Fo, (D) Ti vs. Ca. For the Valmaggia pipe, ‘center’ and ‘rim’ refer to samples collected from the inner and outer portions of the pipe. (E) Primitive mantle normalized trace element spidergram showing the compositional averages for each pipe sample. Primitive mantle values are from McDonough and Sun (1995).

Figure 5 –

(A) Primitive mantle-normalized concentrations of minor and trace elements, and (B) C1 chondrite-normalized rare earth element concentrations in orthopyroxene from the pipes. Shown are the compositional averages for each pipe sample. For the Valmaggia pipe, ‘center’ and ‘rim’ refer to samples collected from the inner and outer portions of the pipe. Primitive mantle values are from McDonough and Sun (1995).

Figure 6 –

(A) Primitive mantle-normalized concentrations of minor and trace elements, and (B) C1 chondrite-normalized rare earth element concentrations in pargasite from the pipes. Shown are the compositional averages for each pipe sample. For the Valmaggia pipe, ‘center’ and ‘rim’ refer to samples collected from the inner and outer portions of the pipe. Primitive mantle values are from McDonough and Sun (1995).

Figure 7 –

(A) Primitive mantle-normalized trace element concentrations of minor and trace elements, and (B) C1 chondrite-normalized rare earth element concentrations in phlogopite from the pipes. Shown are the compositional averages for each pipe sample. For the Valmaggia pipe, ‘center’ and ‘rim’ refer to samples collected from the inner and outer portions of the pipe. Primitive mantle values are from McDonough and Sun (1995).

Figure 8 –

(A) Primitive mantle-normalized concentrations of minor and trace elements, and (B) C1 chondrite-normalized rare earth element concentrations in bulk rock samples from the pipes. The plots show average compositions for each pipe compiled using data from this work and Garuti et al. (2001). Primitive mantle and C1-chondrite values are from McDonough and Sun (1995).

Figure 9 –

U-Pb Concordia plot of results from chemical abrasion isotope dilution TIMS analyses of zircons from the Valmaggia pipe (sample VMG-2).

Figure 10 –

Liquidus equilibria in part of the pseudo-system CaAl₂O₄-quartz-diopside-olivine based on experimental data for hydrous olivine basalt (Adam et al., 2007) and basanite melts (Adam and Green, 2006). The projection is from diopside and the relationships are for a diopside-saturated system. Also plotted are bulk-rock compositional data for the individual pipes (green squares), average mineral compositions for the individual pipes (white squares; data for plagioclase from Garuti et al (2001), together with an average ocean-island-basalt (blue square, based on data from the GEOROC data base). It is noted that only amphibole shows notable major element variation, whereas the other pipe mineral averages mostly plot on top of another. P1 = Peritectic 1, amph = amphibole, opx = orthopyroxene, plag = plagioclase. All compositions have been re-cast as CMAS components following the system of O’Hara (1968).

Figure 11 –

(A) Minor and trace element contents of a parental magma calculated with the assumption that no inter-cumulus melt was retained in the pipes (F=0), and melt compositions in equilibrium with pipe amphiboles. The partition coefficients used in calculations are from Adam et al., (2007) with mineral compositions from Appendix 2 and bulk-rock data from Garuti et al., (2001). The compositions of selected lamproites (Turner et al., 1999; Peccerillo and Martinotti, 2006) and an average ocean-island basalt (GEOROC database) are shown for comparison. (B) Minor and trace element contents of parental magmas calculated with the assumption that 10% (F=10) or 20% (F=20) inter-cumulus melt was retained in the pipes. The partition coefficients used in calculations are from Adam et al. (2007) [amphibole], Adam and Green (2006) [orthopyroxene and olivine], Kiseeva and Wood (2013) [sulfide melt], and Blundy and Wood (1991) [plagioclase], with bulk-rock data from Garuti et al. (2001). The compositions of arc-like basalts from Meshkan (Shabanian et al., 2012) and an average ocean-island basalt (GEOROC database) are shown for comparison.

Figure 12–

Plot of V/Sc vs. Fo in olivines from the pipes. For the Valmaggia pipe average mineral compositions for the individual pipes (data for plagioclase from, ‘center’ and ‘rim’ refer to samples collected from the inner and outer portions of the pipe. Low V/Sc ratios (<1–2) suggest oxidizing conditions during olivine crystallization, whereas reducing conditions would result in higher ratios.

Figure 13 –

Plots of Cs/Rb vs. Zr/Nb for phlogopite from the pipes. For the Valmaggia pipe, ‘center’ and ‘rim’ refer to samples collected from the inner and outer portions of the pipe.

Figure 14 –

Schematic illustrations of the geodynamic evolution that facilitated the genesis of the pipes. (A) Early oceanic subduction in the Eo-Variscan (420 – 380 Ma) initiating metasomatism of the lithospheric mantle. (B, C) Continental collision and ongoing metasomatism of the lithospheric mantle in the Meso-Variscan (360 – 310 Ma) creates pods of metasomatized mantle (cf. D). (D) End of the continental collision and beginning of the gravitational collapse in the Neo-Variscan (300 – 280 Ma). (E) Post orogenic collapse, crustal extension and asthenospheric rise cause decompression melting of the lithospheric mantle and initiate the underplating of the continental crust. (F) Further extension and asthenospheric rise cause partial melting of the metasomatized mantle pods, facilitating the intrusion of the pipe magmas into rocks of the upper lithospheric mantle and lower continental crust.