- Email: [email protected]
The case for crust–mantle interaction during silicic magma genesis: The zircon testimony
Goldschmidt Conference Abstracts 2006
Ultrahigh-temperature crustal metamorphism
A313
The case for crust–mantle interaction during silicic magma genesis: The zircon testimony
DAVID E. KELSEY
A.I.S. KEMP1,2, C.J. HAWKESWORTH2, B.A. PATERSON2, G.L. FOSTER2, J.D. WOODHEAD3, J.M. HERGT3, R.J. WORMALD4
Continental Evolution Research Group, School of Earth and Environmental Sciences, University of Adelaide, Australia, 5005 ([email protected]) 1
Ultrahigh-temperature (UHT) metamorphism, characterized by crustal temperatures exceeding 900 °C, is now accepted as a widespread and important style of regional granulite-facies metamorphism. UHT metamorphism occurs in excess of the solidus of most crustal rocks and is typically recognised on the basis of unusual mineral assemblages in Mg-Al-rich rocks, including sapphirine + quartz, osumilite and orthopyroxene + sillimanite. The generation and maintenance of UHT metamorphic conditions is a conundrum in Earth Sciences as it requires the revision of tectonic models and has implications for the mechanical and rheological behaviour of the lithopshere. Two approaches are required in order to understand UHT metamorphism. First is to assess the mineralogical and temporal record of UHT terranes, by deciphering the thermal and physical (i.e. P-T) conditions, and to constrain timescales, for their evolution. The retrieval of P-T conditions and the meaning of geochronological data is hampered somewhat since the extreme temperatures typically exceed the closure temperatures of most isotopic and elemental closure systems. Nevertheless, developments in mineral equilibria modeling and P-T retrieval techniques have allowed for P-T evolution histories to be tightly constrained. Similarly, the timing of zircon growth is becoming better understood through studies investigating REE patterns in minerals coexisting with zircon and through constraints provided by zircon dissolution. Armed with P-T-t and field/geochemical information, the second approach involves investigation of the tectonic setting(s) of UHT metamorphism. The generation of granulites, including UHT granulites, remains in dispute, with extensional (arc accretion) as well as collisional settings proposed. All models invoke the role of either lithospheric mantle or asthenosphere in generating high temperatures in the deep crust. However, this notion must be questioned in the general absence of voluminous synmetamorphic mantle-derived rocks in UHT terranes. The future direction of studies on UHT metamorphism must be towards improving our understanding of the geodynamic and tectonic implications of UHT terranes. doi:10.1016/j.gca.2006.06.1583
School of Earth Sciences, James Cook University, Townsville, Qld 4811, Australia ([email protected]) 2 Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK 3 School of Earth Sciences, University of Melbourne, Melbourne, Vic. 3010, Australia 4 Malvern Gold, 15 Leamington Court, Wells Road, Malvern WR14 4HF, UK The Sr–Nd–O–Pb isotope arrays exhibited by granitic batholiths and their volcanic associates implicate crust–mantle mixing, but this is commonly not corroborated by whole-rock geochemical evidence. The nature of the alleged mixing process, the stage of magmatic evolution at which it occurs, the composition of the end-member components, and the relevance of the process for the generation of Earth’s continental crust all remain contentious. Zircon is a robust witness, since the O and Hf isotope compositions of its growth zones record the crystallisation history of the host magma, and potentially fingerprint the involvement of mantle-derived agents, or the reworking of pre-existing crust. Such evidence can now be readily summoned by precise in situ microanalysis using new generation ion microprobes and laser ablation systems. Here, we here examine and evaluate the zircon testimony with reference to the classic granitic/volcanic rocks of eastern Australia, where the case for large scale magma mixing has been keenly debated for three decades. We find that zircons in I-, Sand A-type granites and their enclaves indeed preserve the legacy of isotopically open-system behaviour, but that the zircon response has different guises depending on the intrinsic circumstances of each magmatic system. Large variations in O–Hf isotope and trace element compositions indicate that zircons of a single rock have sampled very different melt compositions, suggesting physical accumulation from an evolving magma body, and having implications for pluton construction in the upper crust. The similar zircon isotope systematics of rocks with markedly different silica contents suggests that the processes controlling isotope variations in minerals were decoupled from those that determine bulk rock geochemistry. Linking these observations, we conclude that the isotopic properties of the Lachlan granites are set by open-system differentiation of parental magmas in the deep crust, whereas within-suite compositional differences reflect shallow-level crystal–liquid sorting processes. The extent to which forensic zirconology can reinvigorate studies of granitic rocks, and help stitch the plutonic–volcanic connection, will be discussed. doi:10.1016/j.gca.2006.06.633
Ultrahigh-temperature crustal metamorphism
A313
The case for crust–mantle interaction during silicic magma genesis: The zircon testimony
DAVID E. KELSEY
A.I.S. KEMP1,2, C.J. HAWKESWORTH2, B.A. PATERSON2, G.L. FOSTER2, J.D. WOODHEAD3, J.M. HERGT3, R.J. WORMALD4
Continental Evolution Research Group, School of Earth and Environmental Sciences, University of Adelaide, Australia, 5005 ([email protected]) 1
Ultrahigh-temperature (UHT) metamorphism, characterized by crustal temperatures exceeding 900 °C, is now accepted as a widespread and important style of regional granulite-facies metamorphism. UHT metamorphism occurs in excess of the solidus of most crustal rocks and is typically recognised on the basis of unusual mineral assemblages in Mg-Al-rich rocks, including sapphirine + quartz, osumilite and orthopyroxene + sillimanite. The generation and maintenance of UHT metamorphic conditions is a conundrum in Earth Sciences as it requires the revision of tectonic models and has implications for the mechanical and rheological behaviour of the lithopshere. Two approaches are required in order to understand UHT metamorphism. First is to assess the mineralogical and temporal record of UHT terranes, by deciphering the thermal and physical (i.e. P-T) conditions, and to constrain timescales, for their evolution. The retrieval of P-T conditions and the meaning of geochronological data is hampered somewhat since the extreme temperatures typically exceed the closure temperatures of most isotopic and elemental closure systems. Nevertheless, developments in mineral equilibria modeling and P-T retrieval techniques have allowed for P-T evolution histories to be tightly constrained. Similarly, the timing of zircon growth is becoming better understood through studies investigating REE patterns in minerals coexisting with zircon and through constraints provided by zircon dissolution. Armed with P-T-t and field/geochemical information, the second approach involves investigation of the tectonic setting(s) of UHT metamorphism. The generation of granulites, including UHT granulites, remains in dispute, with extensional (arc accretion) as well as collisional settings proposed. All models invoke the role of either lithospheric mantle or asthenosphere in generating high temperatures in the deep crust. However, this notion must be questioned in the general absence of voluminous synmetamorphic mantle-derived rocks in UHT terranes. The future direction of studies on UHT metamorphism must be towards improving our understanding of the geodynamic and tectonic implications of UHT terranes. doi:10.1016/j.gca.2006.06.1583
School of Earth Sciences, James Cook University, Townsville, Qld 4811, Australia ([email protected]) 2 Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK 3 School of Earth Sciences, University of Melbourne, Melbourne, Vic. 3010, Australia 4 Malvern Gold, 15 Leamington Court, Wells Road, Malvern WR14 4HF, UK The Sr–Nd–O–Pb isotope arrays exhibited by granitic batholiths and their volcanic associates implicate crust–mantle mixing, but this is commonly not corroborated by whole-rock geochemical evidence. The nature of the alleged mixing process, the stage of magmatic evolution at which it occurs, the composition of the end-member components, and the relevance of the process for the generation of Earth’s continental crust all remain contentious. Zircon is a robust witness, since the O and Hf isotope compositions of its growth zones record the crystallisation history of the host magma, and potentially fingerprint the involvement of mantle-derived agents, or the reworking of pre-existing crust. Such evidence can now be readily summoned by precise in situ microanalysis using new generation ion microprobes and laser ablation systems. Here, we here examine and evaluate the zircon testimony with reference to the classic granitic/volcanic rocks of eastern Australia, where the case for large scale magma mixing has been keenly debated for three decades. We find that zircons in I-, Sand A-type granites and their enclaves indeed preserve the legacy of isotopically open-system behaviour, but that the zircon response has different guises depending on the intrinsic circumstances of each magmatic system. Large variations in O–Hf isotope and trace element compositions indicate that zircons of a single rock have sampled very different melt compositions, suggesting physical accumulation from an evolving magma body, and having implications for pluton construction in the upper crust. The similar zircon isotope systematics of rocks with markedly different silica contents suggests that the processes controlling isotope variations in minerals were decoupled from those that determine bulk rock geochemistry. Linking these observations, we conclude that the isotopic properties of the Lachlan granites are set by open-system differentiation of parental magmas in the deep crust, whereas within-suite compositional differences reflect shallow-level crystal–liquid sorting processes. The extent to which forensic zirconology can reinvigorate studies of granitic rocks, and help stitch the plutonic–volcanic connection, will be discussed. doi:10.1016/j.gca.2006.06.633