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Origin of heterogeneous mafic enclaves by two-stage hybridisation in magma conduits (dykes) below and in granitic magma chambers

Published online by Cambridge University Press:  03 November 2011

W. J. Collins ,
S. R. Richards ,
B. E. Healy  and
P. I. Ellison
W. J. Collins
Affiliation:
School of Geosciences,University of Newcastle, Newcastle, NSW 2308, Australia
S. R. Richards
Affiliation:
School of Geosciences,University of Newcastle, Newcastle, NSW 2308, Australia
B. E. Healy
Affiliation:
School of Geosciences,University of Newcastle, Newcastle, NSW 2308, Australia
P. I. Ellison
Affiliation:
School of Geosciences,University of Newcastle, Newcastle, NSW 2308, Australia
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Abstract

Field, petrographic and geochemical evidence from the K-feldspar megacrystic Kameruka pluton, Lachlan Fold Belt, southeastern Australia, suggests that complex, multicomponent, mafic microgranular enclaves (MME) are produced by two-stage hybridisation processes. Stage 1 mixing occurs in composite dykes below the pluton, as mafic and silicic melts ascend through shared conduits. Pillows formed in these conduits are homogeneous, fine-to medium-grained stage 1 MME, which typically range from basaltic to granitic compositions that plot as a sublinear array on Harker diagrams. Stage 2 hybridisation occurs in the magma chamber when the composite dykes mix with the resident magma as synplutonic dykes. The stage 2 hybrids also form linear chemical arrays and range from basaltic to granodioritic compositions, the latter resembling the more mafic phases of the pluton. Stage 2 MME are distinguished from stage 1 types by the presence of K-feldspar xenocrysts and a more heterogeneous nature: they commonly contain stage 1 enclaves. Subsequent disaggregation and dispersal of stage 2 hybrid synplutonic dykes within the magma chamber produces a diverse array of multi-component MME.

Field evidence for conduit mixing is consistent with published analogue experimental studies, which show that hybrid thermo-mechanical boundary layers (TMBL) develop between mafic and silicic liquids in conduits. A mechanical mixing model is developed, suggesting that the TMBL expands and interacts with the adjacent contrasting melts during flow, producing an increasing compositional range of hybrids with time that are mafic in the axial zone, grading to felsic in the peripheral zones in the conduit. Declining flow rates in the dyke and cooling of the TMBL zones produce a pillowing sequence progressing from mafic to felsic, which explains the general observation of more MME in more silicic hosts.

The property of granitic magmas to undergo transient brittle failure in seismic regimes allows analogies with fractured solids to be drawn. The fracture network in granitic magmas consists of through-going ‘backbone’ mafic and silicic ± composite dykes, and smaller ‘dangling’ granitic dykes locally generated in the magma chamber. Stage 1 hybrids form in composite backbone dykes and stage 2 hybrids form where they intersect dangling dykes in the magma chamber. With subsequent shear stress recovery, the host magma chamber reverts to a visco-plastic material capable of flow, resulting in disaggregation and dispersal of these complex, hybrid synplutonic dykes, and a vast array of double and multicomponent enclaves potentially develop in the pluton.

Keywords

double enclavesLachlan fold beltmagma mixingrheologysynplutonic
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 91 , Issue 1-2: Fourth Hutton Symposium: The Origin of Granites and Related Rocks , 2000 , pp. 27 - 45
Copyright
Copyright © Royal Society of Edinburgh 2000

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References

Barbarin, B. 1988. Field evidence for successive mixing and mingling between the Piolard Diorite and the Saint-Julien-la-Vetre Monzogranite (Nord-Forez, Massif Central, France). Canadian Journal of Earth Sciences 25, 49–59.Google Scholar
Barbarin, B.&Didier, J. 1991. Review of the main hypothesis proposed for the genesis and evolution of mafic microgranular enclaves. In Didier, J.&Barbarin, B. (eds) Enclaves and Granite Petrology, Developments in Petrology 13, 367–74. Amsterdam: Elsevier.Google Scholar
Beams, S. D. 1980.Magmatic evolution of the southeast Lachlan Fold Belt, Australia (unpublished Ph.D. Thesis, La Trobe University, Victoria, Australia).Google Scholar
Bedard, J. 1990.Enclaves from the A-type granite of the Megentic complex, White Mountain magma series: clues to granite magma genesis. Journal of Geophysical Research 95, 17797–819.Google Scholar
Blake, S.&Campbell, I. H. 1986. The dynamics of magma-mixing during flow in volcanic conduits. Contributions to Mineralogy and Petrology 94, 72–81.10.1007/BF00371228Google Scholar
Blake, S.& Koyaguchi, T. 1991. Insights from the magma mixing model from volcanic rocks. In Didier, J.& Barbarin, B. (eds) Enclaves and Granite Petrology, Developments in Petrology 13, 403–13. Amsterdam: Elsevier.Google Scholar
Blundy, J. D.& Sparks, R. S. J. 1992. Petrogenesis of mafic inclusions in granitoids of the Adamello Massif, Italy. Journal of Petrology 33, 1039–104.Google Scholar
Campbell, I. H.& Turner, J. S. 1986. The influence of viscosity on fountains in magma chambers. Journal of Petrology 27, 1–30.Google Scholar
Collins, W. J., Richards, S. R., Healy, B.& Wiebe, R. A. 2000. Granite magma transfer, pluton construction, the role of mafic magmas, and the metamorphic response: southeastern Laehlan Fold Belt, Australia. FP3, 142 pp. Sydney, Australia: Geological Society of Australia.Google Scholar
Cox, S. F. 1999. Deformational controls on the dynamics of fluid flow in mesothermal gold systems. In McCaffrey, K. C., Lonergan, L.& Wilkinson, J. J. (eds) Fractures, Fluid Flow and Mineralization, Geological Society of London Special Publication 155, 123–40. London: Geological Society.Google Scholar
Didier, J. 1973. Granites and their enclaves, Developments in Petrology. Amsterdam: Elsevier.Google Scholar
Dingwell, D. B. 1997. The brittle-ductile transition in high level granitic magmas: material constraints. Journal of Petrology 38, 1635–44.Google Scholar
Fernandez, C.& Barbarin, B. 1991. Relative rheology of coeval mafic and felsic magmas: Nature of resulting interaction processes. Shape and mineral fabrics of mafic microgranular enclaves. In Didier, J.& Barbarin, B. (eds) Enclaves and Granite Petrology, Developments in Petrology 13, 263–76. Amsterdam: Elsevier.Google Scholar
Frost, T. P.& Mahood, G. A. 1987. Field, chemical and physical constraints on mafic-fclsic magma interaction in the Lamark Granodioritc, Sierra Nevada, California. Geological Society of America Bulletin 99, 272–91.Google Scholar
Harker, A. 1904. The Tertiary igneous rocks of Skye. Memoir of the Geological Society of Scotland.Google Scholar
Hibbard, M. J. 1991. Textural anatomy of twelve magma-mixed granitoid systems. In Didier, J.& Barbarin, B. (eds) Enclaves and Granite Petrology, Developments in Petrology 13, 158–170. Amsterdam: Elsevier.Google Scholar
Holden, P., Halliday, A. N., Stephens, W. E.& Henney, P. J. 1991. Chemical and isotopic evidence for major mass transfer between mafic enclaves and felsic magma. Chemical Geology 92, 135–52.Google Scholar
Kouchi, A.& Sunagawa, I. 1983. Mixing basaltic and dacitic magmas by forced convection. Nature 304, 527–8.Google Scholar
Koyaguchi, T. 1985. Magma mixing in a conduit. Journal of Volcanology and Geothermal Research 25, 365–9.Google Scholar
Koyaguchi, T.& Blake, S. 1991. Origin of mafic enclaves: Constraints on the magma mixing model from fluid dynamic experiments. In Didier, J.& Barbarin, B. (eds) Enclaves and Granite Petrology, Developments in Petrology 13, 415–29. Amsterdam: Elsevier.Google Scholar
Matthai, S. K.& Roberts, S. G. 1997. Transient versus continuous fluid flow in scismically active faults: an investigation by electric analogue and numerical modelling. In Jamtveit, B.& Yardley, B. W. D. (eds) Fluid Flow and Transport in Rocks: Mechanisms and Effects, 263–92. London: Chapman & Hall.Google Scholar
Metcalf, R. V., Smith, E.,Walker, J. D., Reed, R. C.& Gonzales, E. A. 1995. Isotopic disequilibrium between commingled hybrid magmas: evidence for a two-stage magma mixing-commingling process in the Mt Perkins pluton, Arizona. Journal of Geology 103, 509–27.Google Scholar
Ottino, J. M. 1989. The mixing of fluids. Scientific American 60, 40–49.Google Scholar
Pitcher, W. S. 1991. Synplutonic dykes and mafic enclaves. In Didier, J.& Barbarin, B. (eds) Enclaves and Granite Petrology, Developments in Petrology 13, 383–92. Amsterdam: Elsevier.Google Scholar
Poli, G.& Tommasini, S. 1991. Model for the origin and significance of microgranular enclaves in calcalkaline granitoids. Journal of Petrology 32, 657–66.Google Scholar
Poli, G., Tommasini, S.& Halliday, A. N. 1996. Trace clement and isotopic exchange during acid-basic magma interaction processes. Transactions of the Royal Society of Edinburgh: Earth Sciences 87, 225–32.Google Scholar
Reid, J. B., Evans, O. C.& Fates, D. G. 1983. Magma mixing in granitic rocks of the central Sierra Nevada, California. Earth and Planetary Science Letters 66, 243–61.Google Scholar
Roddick, J. A.& Armstrong, J. E. 1959. Relict dykes in the Coast Mountains near Vancouver, B.C. Journal of Geology 67, 603–13.Google Scholar
Sibson, R. H. 1989. Earthquake faulting as a structural process. Journal of Structural Geology 11, 1–14.Google Scholar
Synder, D., Crambes, C., Tait, S.& Wiebe, R. A. 1997. Magma mingling in dykes and sills. Journal of Geology 105, 75–86.Google Scholar
Sparks, R. S. J.& Marshall, L. A. 1986. Thermal and mechanical constraints on mixing between mafic and silicic magmas. Journal of Volcanology and Geothermal Research 29, 99–124.Google Scholar
Tobisch, O. T., McNulty, B. A.& Vernon, R. H. 1997. Microgranitoid enclave swarms in granitic plutons, central Nevada California. Lithos 40, 321–39.Google Scholar
Vernon, R. H. 1984. Microgranitoid enclaves in granites—globules of hybrid magma quenched in a plutonic environment. Nature 309, 438–9.Google Scholar
Vernon, R. H. 1986. K-feldspar megacrysts in granites—phenocrysts, not porphyroblasts. Earth-Science Reviews 23, 1–63.Google Scholar
Vernon, R. H. 1990. Crystallization and hybridism in microgranitoid enclave magmas: microstructural evidence. Journal of Geophysical Research 95, 17849–59.Google Scholar
Vigneresse, J. L., Barbey, P.& Cuney, M. 1996. Rheological transitions during partial melting and crystallization with application to felsic magma segregation and transfer. Journal of Petrology 37, 1579–600.Google Scholar
Wiebe, R. A. 1996. Mafic–silicic layered intrusions: the role of basaltic injections on magmatic processes and the evolution of silicic magma chambers. Transactions of the Royal Society of Edinburgh: Earth Sciences 87, 233–42.Google Scholar
Wiebe, R. A., Smith, D., Sturm, M., King, E. M.& Seckler, M. S. 1997. Enclaves in the Cadillac Mountain Granite (coastal Maine): Samples of hybrid magma from the base of the chamber. Journal of Petrology 38, 393–423.Google Scholar
Wiebe, R. A.& Collins, W. J. 1998. Depositional features and stratigraphic sections in granitic plutons: implications for the emplacement and crystallization of granitic magma chambers. Journal of Structural Geology 20, 1273–89.Google Scholar
Wilkinson, J J.& Johnson, J. D. 1996. Pressure fluctuations, phase separation, and gold precipitation during seismic fracture propagation. Geology 24, 295–398.Google Scholar