Presentation Information
[O12-P99]Environmental Condition Evaluation of Plutonic to Metamorphic Rocks Using Mineral Zoning
-Evidence from the South Island, New Zealand-
*Daiki Nishikawa1, *Ayumi Yamaguchi1, *Sawa Matsuoka1, *Yura Itaya1, Yukino Tsujimoto1, Kazuma Fujimoto1, Yuki Sekio1, Yuki Tada1, Kota Touji1, Chihiro Wada1 (1. Hyogo Prefectural Himeji Higashi Senior High School)
Keywords:
Oscillatory Zoned Structure,Compositional Zoning,Late-Stage Magmatic Differentiation,Hydorothernal Residual Liquid,Mylonitization,Dissemination
In 2024, amphibole samples taken from diorite and tonalite during an outcrop survey of the Bingi Bingi intrusive complex in southeastern Australia revealed a wavy, millimetre-order microstructure characterized by repeating micrometer-scale bands accompanied by alternating cation substitutions. This discovery is referred to as oscillatory zoning.
Electron probe microanalysis (EPMA) of oscillatory-zoned amphibole samples, together with chemical analyses of coexisting feldspar and oxide minerals, indicate that circulation of hydrothermal residual liquid triggered ion exchanges in crystallized minerals during late stage magmatic differentiation. Geothermobarometric estimates correspond to subsolidus conditions and so the pale green rim of amphibole samples and the oscillatory zoning developed therein are interpreted to have formed through progressive oxidation during subsolidus processes.
Several new questions emerged: (1) If oscillatory zoning in double-chain silicates such as amphibole reflects late-stage magmatic differentiation, might similar structures also occur in single-chain silicates such as pyroxene? (2) Is this phenomenon applicable to intrusive rocks of different ages and regions? (3) Are the oscillatory zoning structures formed by metamorphism, weak metamorphism of plutonic rocks, or ion exchange induced by hydrothermal residual liquid part of continuous processes governed by similar conditions? Do these processes produce similar oscillatory zoning structures?
To address these questions a geological survey in the structurally complex terranes of the Nelson–Motueka area of New Zealand, which exposes part of the Median Batholith including Devonian and Cretaceous diorite and gabbro, was conducted. West of Motueka, portions of the Median Batholith that formed along the Gondwana continental margin during subduction from the Devonian to Early Cretaceous are exposed. In southern Nelson, crystalline limestone, blueschist, and epidote schist of Silurian–Ordovician age occur, intruded by Devonian to Cretaceous plutonic rocks. The western gabbro bodies have experienced strong metamorphism and are mylonitized, whereas Cretaceous diorite occurs to the east. Sandstone units are distributed in southern and northern Motueka and southern Nelson. Conglomerate units are widely distributed along the Tasman Bay. The presence of diorite clasts within the sandstone suggests that the conglomerate was deposited after the formation of metamorphic rocks, plutonic rocks, and sandstone units.
Among the 20 collected rock samples, gabbro shows a high density (3.2 g/cm³) and contains abundant pyrite mineralization. Diorite also contains pyrite aggregates but has a lower density (2.7 g/cm³). Petrographic observations reveal that the gabbro has been mylonitized into metagabbro. Pyrite is abundant, and fragmented amphibole and olivine grains are strongly elongated and aligned. Pyroxene is largely recrystallized into fine-grained aggregates of amphibole, although some rounded relic pyroxene grains partially resorbed by amphibole are preserved. No oscillatory zoning was identified in either the relic pyroxene or the recrystallized amphibole. During subduction and mylontization, minerals recrystallized and equilibrated rapidly, preventing the development of oscillatory zoning.
The diorite experienced weaker metamorphism, or alteration insufficient to cause mylonitization, but pyrite mineralization is present. Oscillatory zoning, interpreted to have formed during late stage magmatic differentiation, is preserved in the rims of partially euhedral amphibole grains. In addition, compositional zoning derived from earlier oscillatory zoning is locally observed in the rim. In relatively weakly metamorphosed diorite, oscillatory zoning in amphibole is preserved and may have formed through circulation of hydrothermal residual liquid during the late magmatic stage, followed by gradual re-equilibration during subsequent weak metamorphism and mineralization. In contrast, compositional zoning in amphibole samples from metamorphic rocks record continuous compositional changes in response to temperature and pressure variations, and the formation process differs from that of oscillatory zoning produced by late-stage hydrothermal residual liquid circulation.
Electron probe microanalysis (EPMA) of oscillatory-zoned amphibole samples, together with chemical analyses of coexisting feldspar and oxide minerals, indicate that circulation of hydrothermal residual liquid triggered ion exchanges in crystallized minerals during late stage magmatic differentiation. Geothermobarometric estimates correspond to subsolidus conditions and so the pale green rim of amphibole samples and the oscillatory zoning developed therein are interpreted to have formed through progressive oxidation during subsolidus processes.
Several new questions emerged: (1) If oscillatory zoning in double-chain silicates such as amphibole reflects late-stage magmatic differentiation, might similar structures also occur in single-chain silicates such as pyroxene? (2) Is this phenomenon applicable to intrusive rocks of different ages and regions? (3) Are the oscillatory zoning structures formed by metamorphism, weak metamorphism of plutonic rocks, or ion exchange induced by hydrothermal residual liquid part of continuous processes governed by similar conditions? Do these processes produce similar oscillatory zoning structures?
To address these questions a geological survey in the structurally complex terranes of the Nelson–Motueka area of New Zealand, which exposes part of the Median Batholith including Devonian and Cretaceous diorite and gabbro, was conducted. West of Motueka, portions of the Median Batholith that formed along the Gondwana continental margin during subduction from the Devonian to Early Cretaceous are exposed. In southern Nelson, crystalline limestone, blueschist, and epidote schist of Silurian–Ordovician age occur, intruded by Devonian to Cretaceous plutonic rocks. The western gabbro bodies have experienced strong metamorphism and are mylonitized, whereas Cretaceous diorite occurs to the east. Sandstone units are distributed in southern and northern Motueka and southern Nelson. Conglomerate units are widely distributed along the Tasman Bay. The presence of diorite clasts within the sandstone suggests that the conglomerate was deposited after the formation of metamorphic rocks, plutonic rocks, and sandstone units.
Among the 20 collected rock samples, gabbro shows a high density (3.2 g/cm³) and contains abundant pyrite mineralization. Diorite also contains pyrite aggregates but has a lower density (2.7 g/cm³). Petrographic observations reveal that the gabbro has been mylonitized into metagabbro. Pyrite is abundant, and fragmented amphibole and olivine grains are strongly elongated and aligned. Pyroxene is largely recrystallized into fine-grained aggregates of amphibole, although some rounded relic pyroxene grains partially resorbed by amphibole are preserved. No oscillatory zoning was identified in either the relic pyroxene or the recrystallized amphibole. During subduction and mylontization, minerals recrystallized and equilibrated rapidly, preventing the development of oscillatory zoning.
The diorite experienced weaker metamorphism, or alteration insufficient to cause mylonitization, but pyrite mineralization is present. Oscillatory zoning, interpreted to have formed during late stage magmatic differentiation, is preserved in the rims of partially euhedral amphibole grains. In addition, compositional zoning derived from earlier oscillatory zoning is locally observed in the rim. In relatively weakly metamorphosed diorite, oscillatory zoning in amphibole is preserved and may have formed through circulation of hydrothermal residual liquid during the late magmatic stage, followed by gradual re-equilibration during subsequent weak metamorphism and mineralization. In contrast, compositional zoning in amphibole samples from metamorphic rocks record continuous compositional changes in response to temperature and pressure variations, and the formation process differs from that of oscillatory zoning produced by late-stage hydrothermal residual liquid circulation.