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  • AGES 2017 Proceedings, NT Geological Survey

    68

    The geological evolution of the Arnhem Province: implications for craton-scale correlations Jo A Whelan1,2, Barry L Reno1, Anett Weisheit1, Stefan Kraus1,3, Natalie Kositcin4, Jonathan D Woodhead4, Roland Maas5 and Richard A Armstrong6

    © Northern Territory Government March 2017. Copying and redistribution of this publication is permitted but the copyright notice must be kept intact and the source attributed appropriately.

    Introduction

    The Halls Creek and Pine Creek orogens at the northern margin of the North Australian Craton (NAC) preserve evidence of collisional orogenesis at ca 1875–1855 Ma (Figure 1). Contemporaneous with this orogenesis, the Arnhem Province, ~300 km east of the Pine Creek Orogen, underwent high-grade metamorphism, melt generation and granite emplacement. Although the timing of geological events recorded in the Arnhem Province is similar, there are significant differences in the metamorphic, igneous and deformation history from that of the two orogens to the west.

    The Northern Territory Geological Survey (NTGS) has conducted a surface geological mapping program in the Arnhem Province to characterise the depositional, metamorphic, structural and magmatic evolution of the province. The results of this study have been applied to resolve the links between the Arnhem Province and the polymetallic Pine Creek and Halls Creek orogens in an effort to place the Arnhem Province into the wider context of the tectonic events occurring at the northern margin of the NAC during the Palaeoproterozoic.

    The Arnhem Province comprises a succession of metasedimentary and metaigneous rocks that have experienced varying degrees of metamorphism and deformation and have preserved evidence for multiple phases of granite generation and emplacement. The Arnhem Province is exposed in three main areas (Figure 2):

    1. Coastal and inland outcrops between Nhulunbuy and Cape Shield (Figure 2a, b): This region preserves migmatitic metasedimentary rocks that show evidence of a protracted period of metamorphism that resulted in multiple phases of partial melt formation, migration, and accumulation into plutons of S-type granite. Fayalite-bearing A-type granite was emplaced around 30 million years after the metamorphic cycle.

    2. Poorly exposed outcrop at the base of the Coast Range and on islands in the Blue Mud Bay area (Figure 2a, b): This region preserves a greenschist-facies sequence of siliciclastic metasedimentary rocks overlying S-type granites and metasedimentary rocks; all are intruded by the same suite of fayalite-bearing granites as location (1).

    3. The structurally bound Mirarrmina Inlier within the Mitchell Range ~100 km west of the main area of outcropping Arnhem Province: This region’s exposures comprise variably deformed S-type granitic rocks juxtaposed with a volcano-sedimentary sequence of the Spencer Creek Group, McArthur Basin (Figure 2a).

    1 Northern Territory Geological Survey, GPO Box 4550, Darwin NT 0801, Australia

    2 Email: jo.whelan@nt.gov.au 3 Now at IFB Eigenschenk GmbH, Deggendorf, Germany 4 Geoscience Australia, Canberra, Australia 5 School of Earth Sciences, University of Melbourne, Australia 6 Research School of Earth Sciences, The Australian National

    University, Canberra, Australia.

    N T

    Q LD

    N T

    W A

    Palaeo-Mesoproterozoic Orogens ArchaeanPalaeo-Mesoproterozoic Basins

    domain boundaryinterpreted extent of North Australian Craton

    Halls Creek

    Orogen

    Birrindudu Basin

    Warramunga Province

    Tomkinson Province

    Davenport Province

    Aileron Province

    Warumpi Province

    South Nicholson

    Basin Lawn HillPlatform

    Eastern Succession

    Kalkadoon- Leichhardt

    Belt

    Western Succession

    Georgetown Inlier

    Coen Inlier

    Murphy Inlier

    Pine Creek Orogen

    Nimbuwah Domain

    Birrindudu Basin

    TANAMI

    REGION

    REGION

    REGION

    MOUNT

    ISA

    INLIER

    TENNANT

    ARUNTA

    King Leopold Orogen

    Halls Creek

    Orogen

    Carr-Boyd Basin

    Speewah Basin

    Bastian Basin Kimberley Basin

    Kimberley Basin

    Bastian Basin McArthur Basin

    Fitzmaurice Basin Arnhem Province

    NT

    QLDWA

    126°124°

    12°

    14°

    16°

    18°

    20°

    22°

    128° 130° 132° 134° 136° 138° 140° 142° 144°

    0 150 300 km

    Figure 2

    A17-074.ai

    Figure 1. Geological map of the North Australian Craton showing the distribution of Neoarchaean basement, Palaeo–Mesoproterozoic basement and basin.

  • 69

    AGES 2017 Proceedings, NT Geological Survey

    a

    0 50 km

    0 10 20 km

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    normal fault

    fault high-anglereverse fault fault, inferred

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    Sh er

    id an

    Ba th

    Ko ol

    at on

    g

    G al

    i

    Fa ul

    t

    Fa ul

    t Fa ul

    t

    Fa ul

    t Fa

    ul t

    Fa ul

    t

    Fa ul

    t

    Fa ul

    t

    Fa ul

    t Fa

    ul t

    Fa ul

    t

    R an

    ge

    C oa

    st R

    an ge

    Le la

    M itc

    he ll

    Ra ng

    es

    R ita

    ra ng

    o

    G w

    ak ur

    a Fr

    ed er

    ic k

    a

    Mirarrmina Inlier

    b

    Coast Range

    Blue Mud Bay

    b

    Palaeogene–Neogene Quaternary

    Cenozoic McArthur Basin A-type granite Grindall Formation S-type granite Melville Bay Metamorphics

    A 17

    -0 75

    .a i

    A17-075.ai

    1

    2

    3

    CALEDON BAY

    Gooninnah Island

    BLUE MUD BAY

    Cape Shield

    GULF OF CARPENTARIA

    NHULUNBUY

    136°40' 136°50' 137°136°30'136°25'

    -12°10'

    -12°00'

    -12°20'

    -12°30'

    -12°40'

    -12°50'

    -13°00'

    -13°10'

    -13°20'

    -13°30'

    YIRRKALAMELVILLE BAY

    136°30' 136°40'136°20'136°10'136°00'

    Figure 2. (a) Location of areas of outcrop 1, 2 and 3 of the Arnhem Province. (b) Generalised geology map of the Arnhem Province showing the distribution of stratigraphic units in area 1.

  • AGES 2017 Proceedings, NT Geological Survey

    70

    Geological mapping at 1:100 000 scale was undertaken in all of these areas. Petrographic studies, whole-rock geochemistry and Nd isotope data, U–Pb zircon and monazite chronology, in situ zircon Hf–O isotope data, and zircon, monazite and garnet trace-element chemistry were used to augment field observations. This submission presents results of this work, and includes a summary of the geological evolution of the Arnhem Province.

    ca 1900–1880 Ma sedimentation

    The oldest rocks in the Arnhem Province are the Melville Bay Metamorphics, a sequence of metasedimentary migmatitic gneisses comprising metapelite, metapsammite, quartzite, and minor calc-silicate and metacarbonate rocks. These rocks are poorly exposed and only recognised along the coast in the main area of outcrop (Figure 2b). The presence of calc-silicate and metacarbonate rocks suggests the sedimentary precursors were deposited in a shallow water setting.

    The Melville Bay Metamorphics contain dominant detrital zircon components ranging from ca 2300–1900 Ma with subordinate populations ranging back to ca 2609 Ma (Kosticin et al in prep). The maximum depositional age of the succession is difficult to constrain due to high temperature metamorphism shortly after sediment deposition; however, deposition must have occurred between the youngest confirmed detrital zircon at ca 1900 Ma and the oldest metamorphic ages of ca 1880 Ma (Kositcin et al in prep, Reno et al in prep).

    ca 1880–1862 Ma high-thermal gradient metamorphism, migmatite formation, deformation and S-type magmatism

    Following sedimentation, the protoliths to the Melville Bay Metamorphics were metamorphosed to granulite-facies conditions resulting in migmatite formation, melt migration

    (Figure 3) and pooling, and the emplacement of voluminous S-type granites. Pseudosections constructed for samples of the Melville Bay Metamorphics model the highest recorded pressures of 0.6–0.8 GPa at temperatures >825°C. Monazite and zircon, interpreted to have grown in the earliest preserved assemblage, record the highest pressure–temperature phase of the metamorphic cycle at ca 1880 Ma. The Melville Bay Metamorphics experienced cooling and decompression to ~0.4 GPa and ~750°C before cooling below the solidus of ~670°C at pressures >~0.3 GPa. The high thermal gradient conditions in the Melville Bay Metamorphics are consistent with metamorphism in an extensional environment such as a back-arc basin.

    S-type plutons emplaced throughout the Arnhem Province between ca 1880 Ma and ca 1862 Ma are interpreted to have been derived from the partial melting of the Melville Bay Metamorphics, suggesting multiple phases of melt extraction from the Melville Bay Metamorphics during a ca 20 million year supersolidus metamorphic cycle. These S-type granites contain abundant cm-scale garnet that both include cordierite and are mantled by cordierite; the garnets are chemically and petrographically identical to cordierite-mantled garnet occurring as peritectic phases within the M