Zircon chemistry and magma mixing, SE China: In-situ analysis of Hf isotopes, Tonglu and Pingtan igneous complexes

Phanerozoic tectonics of the South China Block: Key observations and controversies

Using hafnium and oxygen isotopes in zircons to unravel the record of crustal evolution

Granite is one of the most important components of the continental crust on our Earth; it thus has been an enduring studied subject in geology. According to present knowledge, granite shows a great deal of heterogeneity in terms of its texture, structure, mineral species and geochemical compositions at different scales from small dike to large batholith. However, the reasons for these variations are not well understood although numerous interpretations have been proposed. The key point of this debate is whether granitic magma can be effectively differentiated through fractional crystallization, and, if so, what kind of crystallization occurred during the magmatic evolution. Although granitic magma has high viscosity because of its elevated SiO2 content, we agree that fractional crystallization is effectively processed during its evolution based on the evidence from field investigation, mineral species and its chemical variations, and geochemical compositions. These data indicate that crystal settling by gravitation is not the only mechanism dominating granitic differentiation. On the contrary, ﬂow segregation or dynamic sorting may be more important. Accordingly, granite can be divided into unfractionated, fractionated (including weakly fractionated and highly fractionated) and cumulated types, according to the differentiation degree. Highly fractionated granitic magmas are generally high in primary temperature or high with various volatiles during the later stage, which make the fractional crystallization much easier than the common granitic melts. In addition, effective magmatic differentiation can be also expected when the magma emplaced along a large scale of extensional structure. Highly fractionated granitic magma is easily contaminated by country rocks due to its relatively prolonged crystallization time. Thus, granites do not always reflect the characteristics of the source areas and the physical and chemical conditions of the primary magma. We proposed that highly fractionated granites are an important sign indicating compositional maturity of the continental crust, and they are also closely related to the rare-elemental (metal) mineralization of W, Sn, Nb, Ta, Li, Be, Rb, Cs, REEs, etc.

Highly fractionated granites: Recognition and research

At low temperatures (<750 °C at moderate to high crustal pressures), the production of sufficient melt to reach the melt connectivity transition (∼7 vol%), enabling melt drainage, requires an influx of aqueous fluid along structurally controlled pathways or recycling of fluid via migration of melt and exsolution during crystallization. At higher temperatures, melting occurs by fluid-absent reactions, particularly hydrate-breakdown reactions involving micas and/or amphibole in the presence of quartz and feldspar. These reactions produce 20–70 vol%, melt according to protolith composition, at temperatures up to 1000 °C. Calculated phase diagrams for pelite are used to illustrate the mineralogical controls on melt production and the consequences of different clockwise pressure-temperature ( P - T ) paths on melt composition. Preservation of peritectic minerals in residual granulites requires that most of the melt produced was extracted, implying a flux of melt through the suprasolidus crust, although some may be trapped during transport, as recorded by composite migmatite-granite complexes. Peritectic minerals may be entrained during melt drainage, consistent with observations from leucosomes in migmatites, and dissolution of these minerals during ascent may be important in the evolution of some crustal magmas. Since siliceous melt wets grains, suprasolidus crust may become porous at only a few volume % melt, as evidenced by microstructures in residual migmatites in which quartz or feldspar pseudomorphs form after melt films and pockets. With increasing melt volume and decreasing effective pressure, assuming the residue is able to deform and compact, the source becomes permeable at the melt connectivity transition. At this threshold, a change from distributed shear-enhanced compaction to localized dilatant shear failure enables melt segregation. The result is a highly permeable vein network that allows transfer of melt to ascent conduits at the initiation of a melt-extraction event. Melt is drained from the anatectic zone via several extraction events, consistent with evidence for incremental construction of plutons from multiple batches of magma. Buoyancy-driven magma ascent occurs via dikes in fractures or via high-permeability zones controlled by tectonic fabrics; the way in which these features relate to compaction and the generation of porosity waves is discussed. Emplacement of laccoliths (horizontal tabular intrusions) and wedge-shaped plutons occurs around the ductile-to-brittle transition zone, whereas steep tabular sheeted and blobby plutons represent back freezing of melt in the ascent conduit or lateral expansion localized by instabilities in the magma–wall-rock system, respectively.

/pdf/granite-from-genesis-to-emplacement-2as9orvmx6.pdf

Granite: From genesis to emplacement

A hybrid origin for Lachlan S-type granites: the Murrumbidgee Batholith example

Detailed field evidence indicates that the Kameruka Suite plutons of the Bega Batholith, eastern Australia, grew by crystal accumulation on the floor of a magma chamber. Depositional features in the plutons, including mafic enclave channels, asymmetric enclave pillows and exotic rafts, load casts and flame structures, and graded and trough cross-beds, indicate that the pluton built progressively upward. The general eastward dip of depositional features in the main pluton implies a lower western and upper eastern contact, consistent with a basal granite–migmatite contact in the west and a sharp hornfelsic sidewall contact in the east. Mafic, felsic and composite dykes, most common near and below the basal western contact, are interpreted as conduits for magma chamber replenishment and imply open-system behaviour during pluton construction. Textural relations are also consistent with an open-system, cumulate origin. Typically, centimetre-scale grains of quartz, plagioclase and megacrystic alkali feldspar form a touching framework with interstices filled with smaller biotite flakes and smaller intercumulus quartz and feldspar crystals. Alkali feldspar megacrysts vary from euhedral and unzoned, to mantled and partially replaced by plagioclase, to ovoid and completely pseudomorphed by quartz–albite aggregates. The common occurrence of mantled and pseudomorphed alkali feldspar in mafic enclaves, and in hybrid tonalitic rocks forming the matrix to enclave swarms, suggests that replacement or resorption of granitic primocrysts was associated with mafic replenishments. The occurrence of all megacryst types at outcrop scale implies extended alkali feldspar crystallization in different parts of the chamber, thorough stirring during, or after, periodic replenishment, and final settling in a cumulate mush. The bulk composition of the cumulate mush, represented by granodiorite, cannot represent the emplaced magma. Compositional variation can be modelled by variable degrees of crystal accumulation from a parental, silica-rich melt represented by the silicic dykes. As dykes periodically fed the magma chamber, crystals accumulated on the floor, and more evolved melts probably erupted from its roof. Thus, the average composition of the magma, and the cumulus minerals, may have remained relatively constant, and the sublinear chemical trends that typify the Kameruka Suite simply reflect differing proportions of melt and cumulate material. Sublinear chemical trends can also be explained by a restite model; however, the distinctive Ba, light rare earth element and Zr spikes at high silica can be explained only by a cumulate model, which also explains why the low-silica granites of the suite share the same chemical characteristics as the high-silica granites.

Replenishment, crystal accumulation and floor aggradation in the Megacrystic Kameruka Suite, Australia

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.

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

Field Trip to the Base of the Bega Batholith - An Overview

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A hybrid origin for Lachlan S-type granites: the Murrumbidgee Batholith example

Replenishment, crystal accumulation and floor aggradation in the Megacrystic Kameruka Suite, Australia

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

Field Trip to the Base of the Bega Batholith - An Overview