In the last five years, the view that the northern Australian Early to Middle Proterozoic mobile belts were generated in intra-continental rift settings has become
increasingly popular with Australian geologists (review by Etheridge et al., 1987b). In
the Mount Isa Inlier, evidence for intra-continental rifting was initially based on
lithofacies in the Western Fold Belt (Glikson et al., 1976; Derrick, 1982). Blake et al.
(1985) and Blake (1986 ,1 9 8 7 ) considered the history o f the Mount Isa Inlier as entirely ensialic with two or three successive rift events. This is supported by the chem ical signatures o f dolerites o f various ages in the inlier, which, although they are generally
low in Ba, Rb, Sr and K2O, are comparable to modem-day continental tholeiites (Ellis &
2: Tectonic evolution 52
Episodic extensional tectonics have affected the central and eastern parts of the Mount Isa Inlier (Table 2.1). The first recognizable rifting event to affect the inlier (Dei) resulted in the deposition of coarse clastic sediments and felsic volcanics of the Argylla Formation around 1780 Ma. (Blake et al., 1985; Blake, 1986, 1987), and, as argued in this chapter, possibly the lower SCG. Ongoing crustal extension led to a change from felsic volcanism to mafic upper crustal igneous processes during the deposition of the Marraba Volcanics and Mitakoodi Quartzite, and upper SCG (Fig. 2.10).
As McKenzie (1978) and Beaumont (1978) have shown theoretically, during the period of tectonic quiescense and thermal relaxation that follows continental extension, subsidence occurs and low-energy "sag" sediments are deposited. These are laterally very continuous and lack the extensive extrusive intercalations and syn-sedimentary faults of the rift phase. A blanket of low-energy shelf sediments, including those of the upper Malbon Group, the Overhang Jaspilite, the lower Marimo Slate and maybe the lower Corella Formation has been interpreted as a sag sequence (Blake et cd., 1985; Blake, 1986,1987). This sequence generally lacks extrusives.
The Soldiers Cap Group (and its southwestern correlative the Kuridala Formation) contains the only major body of meta-turbidites in the Mount Isa Inlier. Since the SCG also delineates the eastern margin of the inlier, it has often been argued that this eastern margin represents a Middle Proterozoic continental margin (Plumb & Derrick, 1975; Wilson, 1978; Plumb et al., 1980). Blake et al. (1984) disputed this, arguing in favour of a source rock for the southern SCG towards the east, because, at least in the southern SCG, both the grain size and the feldspar content decrease towards the west (a full review of the pros and cons of the "continental margin" problem can be found in Blake,
1986, and a brief review in chapter 1). To this, one may add and emphasize that
turbidites are not restricted to continental slope environments, although the continuity of the turbidites in the SCG seems to preclude deposition in a small and/or bathymetrically irregular basin. The base of the SCG is exposed south of the studied area: Laing & Beardsmore (1986) and Beardsmore et al. (1988, in press) have documented two lower units, which conformably underlie the SCG. A general progressively upward maturing of the meta-sediments and a marked upward change from bimodal, subaerial, felsic volcanism to subaqueous, mafic volcanism are the typical hallmarks of a rift-and-sag sequence. The SCG contains no MORB-type basalts. The depositional age of the SCG is poorly constrained but it is most likely (in all published correlation schemes) older than the meta-sediments in the Marimo-Staveley Block. Thus, there is no basis for a
palinspastic reconstruction, which incorporates an eastward progressive deepening. More westwards, the Marimo-Staveley Block lithologies overlie those of the Mitakoodi Anticlinorium, so here too eastward deepening cannot be deduced. This adds to other
evidence that refutes *the continental margin model for the eastern margin o f the Mount Isa Inlier.
A new and spatially restricted phase of magmatic activity was superposed on, and disturbed, the first sag phase: the Corella Formation northeast o f the Mitakoodi
Anticlinorium is intruded by the Burstall Granite of the W onga Batholith (1745ti5 Ma., Page, 1983b), whereas in the Tommy Creek Block, meta-sediments o f the Corella Formation interfinger with ignimbritic sheets (Hill, 1987). It seems likely that this magmatic activity is due to a second pulse of extension. This second extensional event
(Dei) m ust have postdated the deposition of the Corella Formation, which it affects. The absolute maxim um age o f the Corella Formation is 1780 Ma., the U-Pb Zr age o f the underlying Argylla Formation (Page, 1 9 7 8 ,1983a). However, as the Burstall Granite intrudes a large section o f the Corella Formation, and sag sequences like the Corella Formation are deposited in slowly subsiding basins (time-constant for thermal relaxation o f an extended lithosphere is in the order o f 60 Ma.; McKenzie, 1978; Jarvis &
M cKenzie, 1980), it can reasonably be assumed that De2 did not start before 1760 Ma..
De2must have predated =1740 Ma., the recently determined age o f a granite in the zone o f extensional structures in the Mary Kathleen Fold Belt which itself is not affected by these extensional structures (R.W. Page, pers. comm., 1988). Thus, the age o f this second extensional event must fall between 1760 and 1740 Ma., and it interfered with the thermal relaxation o f the first, 1780 Ma. old, extensional e v en t Both purely orthogonal extension and purely orthogonal strike-slip-dominated tectonics (or a hybrid form) at De2
can explain the irregular bathymetric patterns that controlled the deposition o f the upper M arimo Slate, Tommy Creek Block lithologies and Roxmere Quartzite. Dolerites
intruded during this event have, like all other dolerites in the inlier, a continental tholeiitic signature. There is no evidence for the formation of oceanic crust at any stage during
De2*
A third extensional event (D ^) affected the western part o f the -now exposed- inlier, and resulted in the deposition of the 1678±1 Ma. old Carters Bore Rhyolite (Page, 1978), the Surprise Creek Formation, and the 1670 Ma. old M ount Isa Group (Fig. 1.3; Page, 1981). Again, there is no evidence for the formation o f oceanic crust. Thus, an (early, basin-forming) ensialic history fo r the inlier is very strongly supported.
Dci is a local thrust event, recognized in the Marimo-Staveley Block and the Soldiers Cap Group (in the Mitakoodi Anticlinorium it is locally manifested by a slaty cleavage) and between the Mitakoodi Anticlinorium and the Tommy Creek Block. Its movement vector pattem is erratic (compare Bell, 1983; BMR, 1985; Loosveld & Schreurs, Appendix 1; Loosveld, Chapter 3). Tear faults which could geometrically account for the different movement vectors have as yet not been recognized. DC2 is the
2: Tectonic evolution 54
penetrative deformation event in the central eastern Mount Isa Inlier (as in the rest o f the
inlier). DC2 gave rise to the most penetrative map-scale structures. These structures were
formed by coaxial E-W compression and vertical extension, resulting in a plane-strain
shortening o f 35-55%. was a phase o f N-S to NE-SW directed compression,
leading to local refolding and to juxtaposition of tectono-stratigraphic blocks o f different metamorphic grade. The youngest structures are kinks and faults.
The eastern part o f the Mount Isa Inlier is a poly-metamorphic terrane. Peak-
metamorphism is syn-D d to pre-DC2. Crustal thickening during DC2 was, however, not
accompanied by high-P facies metamorphism and andesitic volcanism, but by low-P
facies metamorphism. It thus appears that the basin-modifying tectonics o f the inlier,
Del, Dc2, and Dc3, were also characterized by intra-continental processes, rather than by subduction-related processes. In chapters 3 and 4, these compressional events are further described and discussed.
A problem which needs to be addressed by geodynamicists now is the one o f the compressional driving force, in the absence o f preceding formation o f oceanic crust. In the plate tectonics theory, extension and the formation o f oceanic crust is logically followed by cooling o f the oceanic lithosphere and subsequent destruction o f this lithosphere. Continental collision may result No such oceanic crust appears to have
formed in the Proterozoic terranes o f Australia (review by Etheridge et al.y 1987b).
Moreover, in the following chapters (Chapter 3 & 4), the timegap between the
extensional events and the major compression (Dc2) will be shown to exceed 120 Ma., so
THE STRUCTURAL!MET AMORPHIC
FRAMEWORK OF THE CENTRAL
SOLDIERS CAP GROUP BELT
LOOSVELD
55
THE STRUCTURAL/METAMORPHIC FRAMEWORK OF THE
CENTRAL SOLDIERS CAP GROUP BELT
3.1 INTRODUCTION
In this chapter, the relation between the structural and metamorphic histories of the central Soldiers Cap Group belt will be discussed. Specifically, the synchroneity of the growth of low-P facies metamorphic assemblages with DC2, the penetrative, regional deformation event, will be demonstrated. In chapter 4, a general discussion of the possible tectono-magmatic models that may explain low-P facies metamorphism during a phase of crustal thickening will follow.
The structure of the western and central parts of the Mount Isa Inlier is generally characterized by upright, N-S trending folds, widely attributed to Dc2 (Derrick et al.,
1977c; Bell, 1983; Lister et a/., 1986b; Winsor, 1984, 1985,1986; Loosveld &
Schreurs, Appendix 1). In chapter 2 (and also by Donchak et al., 1983; Hill, 1987), it was argued that the characteristics of DC2 in the three tectono-stratigraphic domains of the eastern part of the inlier, including the Soldiers Cap Group belt (Fig. 3.1), are similar to those in the central and western parts. Consensus on the pre-Dc2 history, however, is still far-off. Several different types of early structures have been described west of the
Pilgrim Fault/Ballara-Corella River Fault Zone (Fig. 3.1). Passchier (1986a) interpreted extensional detachment-like geometries around the Alligator Syncline (Fig. 3.1); a penetrative (extensional?) fabric in the Wonga Granite also predates Dc2 (Pearson et al.,
1987; Oliver etal., in prep.). Bell (1983) interpreted a huge thrust duplex with marginal synclines for the area north of Mount Isa township, and Loosveld & Schreurs (Appendix
1) attributed an imbricate stack of thrust sheets between the townships of Mount Isa and Mary Kathleen to Dcl. The temporal and spatial relations between these pre-DC2 structures are not clearly established.
In the eastern part of the inlier, i.e. east of the Pilgrim Fault, the situation is markedly different, and even more obscure. Not only are pre-DC2 structures poorly documented and correlated, the nature of DC2 itself is also a matter of discussion. Dc2 is more varied in orientation and only very few pre-Dc2 structures have been documented and some of these have dubious status. Map-scale pre-Dc2 structures dominate an E-W strip, centred on the Cloncurry township. West of Cloncurry, where the zone is about 5 km wide, it comprises highly non-cylindrical folds and strong LS-fabrics (Hill, 1987). East of Cloncurry, a 10 km wide strip is characterized by tight to isoclinal, upright, E-W trending folds, which have been attributed to Dcl (Glikson and Derrick, 1970). Rybum et al. (in prep.) also describe Dcl thrust planes and Dcl folds east of Cloncurry, but attribute
the E-W folds to D c2. Much o f the confusion springs from the emphasis given to orientation while correlating structures.
S O L D I E R S C A P G R O U P W O N G A G R A N I T E \ Naraku Granite
r\
>
J A T O M M Y CJREEK <t? / V ' M I C R O G R A N I T E " B lockade Block Sybella Granite F I G U R E 3 J MT. I SA f CLONCURRY MA RY K A T H L E E N / j t A L L I G A T O R I S Y N C L I N E F A I R M I L E P R O S P E C T Williams Batholith 1 4 1°F ig . 3 .1 Location o f the Soldiers Cap Group in the Mount Isa Inlier. Broad arrows
represent Dcj thrust movement vectors. Broken arrow represents the southwards
4
3: Deformation/metamorphism 57
Recent work on thin-skinned thrust systems (among others Dahlstrom, 1969; Elliott, 1976a, 1976b; special issue Joum. of Str. GeoL, Vol. 8, No. 3/4, 1986) has shown that one apparently simple compressive event can produce a wide range of structural styles with widely varying trends, and complexity is even greater if the more ductile processes at depth are considered. Careful mapping of cleavage traces provides a means of unravelling such complex geometries (Mitra & Elliott, 1980). This procedure was adopted during my 1986 and 1987 field seasons in the central Soldiers Cap Group belt, and the result demonstrates the simultaneous development of different fold trends that had previously been attributed to two separate fold phases. In brief, the structure, as it is interpreted in this chapter, is one of a refolded fold nappe: an early NW-closing fold nappe with an E-W trending, coaxially shortened frontal/lateral zone (Dcl), folded around a N-S trending, upright syncline (Dc2). Metamorphic grades in the study area are in places sufficiently high for large porphyroblasts to have grown, and micro-structural porphyroblast-foliation relations add to, clarify and confirm field relations. A 1:100 000 scale structural map of the central SCG belt is in appendix 9 (back pocket).