Early Triassic Legoupil Formation in Schmidt Peninsula, Antarctic Peninsula: Provenance and Depositional Settings

0. INTRODUCTION

The Antarctic Peninsula is the largest continental fragment in West Antarctica and the southern Gondwana collage, and contains several major sedimentary basins that formed along the Pacific margin of Gondwana prior to and during continental breakup. Due to its well-developed accretionary, fore-arc, magmatic-arc, and back-arc sequences, the Antarctic Peninsula was initially interpreted as an autochthonous contiguous magmatic arc that formed in response to subduction of Paleo-Pacific oceanic lithosphere beneath southern Gondwana during the Mesozoic and Cenozoic (Bastias et al., 2020; Castillo et al., 2020, 2017, 2016; Nelson and Cottle, 2017; Bradshaw et al., 2012; Fanning et al., 2011; Barbeau et al., 2010; Vaughan and Storey, 2000; Birkenmajer et al., 1997; Birkenmajer, 1992; Loske et al., 1988; Miller, 1983; Harrison et al., 1979; Halpern, 1965). However, recent geophysical and geological studies indicate that the Antarctic Peninsula is a composite magmatic arc that was accreted to the Gondwanan margin during the Cretaceous. The Antarctic Peninsula plays a key role in understanding the tectonic development of the southwest Pacific margin of Gondwana. And the position of the Antarctic Peninsula, prior to the break-up of Gondwana, is obscured due to the poorly exposed pre-Jurassic rocks (Castillo et al., 2020). On the basis of strong similarities between Paleozoic U-Pb geochronology and Hf-O isotopes of igneous and detrital zircons (Castillo et al., 2020, 2017, 2016; Nelson and Cottle, 2017; Bradshaw et al., 2012; Fanning et al., 2011), Antarctic Peninsula is deemed to share a temporal, geochemical and tectonic history with the Patagonia and South American sectors of the arc system and has been concluded to have a considerable geographic overlap with the western flank of Patagonia (Miller, 1983; Harrison et al., 1979). Although plate reconstructions are numerous, varied and poorly constrained (Barbeau et al., 2010), the new data with isotopic zircon data of the Permian–Triassic detrital zircons and Late Permian igneous rocks give a hint for a continuation between Antarctic Peninsula and southern Patagonia, without the need for a greater degree of geographic overlap (Castillo et al., 2020, 2017).

The geology of Antarctic Peninsula is divided into three domains: west domain, central domain and east domain (Vaughan and Storey, 2000). More recently, the central and east domains are concluded to be the autochthony according to the isotopic composition of Triassic igneous and metamorphic rocks from these domains (Bastias et al., 2020). The northern Antarctic Peninsula, belonging to the east domain, largely consist of low to intermediate grade poorly fossiliferous turbiditic and pelagic metasedimentary rocks which are known generally as the Trinity Peninsula Group in Graham Land (Figure 1a). The Trinity Peninsula Group is divided (from north to south) into the Hope Bay Formation, View Point Formation, Legoupil Formation, Charlotte Bay Formation, and Paradise Harbour Formation (Birkenmajer, 1997, 1992). And the Trinity Peninsula Group is an important path to know the Paleozoic subduction-related magmatic arcs.

Figure  1.  The sketch map of northern Antarctic Peninsula after Castillo et al. (2016), the location of Schmidt Peninsula and the simplified geological map of Legoupil Formation after reference Halpern (1965).

DownLoad: Full-Size Img PowerPoint

Many geochronological studies have been done for detrital zircons of Trinity Peninsula Group samples and reveal a large input of Permian magmatic zircons with ages of 290–260 Ma (Castillo et al., 2017, 2016; Fanning et al., 2011; Barbeau et al., 2010; Loske et al., 1988), consistent with sediment derivation entirely from western Gondwana sources. The Legoupil Formation is conventionally considered as a major component of the Trinity Peninsula Group and crops out at Cape Legoupil and on islands along the coast of the Duroch Islands (Halpern, 1965) (Figures 1b, 1c). Many studies have been conducted on the Legoupil Formation; however, there are different opinions about the Legoupil Formation depositional age including Triassic or Cretaceous (Tokarski, 1989; Thomson, 1975; Halpern, 1965) while the tectonic setting of the Legoupil Formation is unclear and might have been an active or passive continental margin (Castillo et al., 2015; Smellie, 1991; Tokarski, 1989). Therefore, further data are required to constrain the tectonic setting and depositional age of the Legoupil Formation.

In this paper, we present zircon U-Pb ages and geochemical data for a package of little-studied siliciclastic rocks on Schmidt Peninsula and try to discuss their geochronological framework, zircon provenance, and tectonic setting. Compared to the known information of Trinity Peninsula Group, we further discuss the affinity of Legoupil Formation and the tectonic setting of Antarctic Peninsula.

1. GEOLOGICAL BACKGROUND AND SAMPLE DESCRIPTIONS

The Legoupil Formation is widespread on the Schmidt Peninsula (Figure 1) and is about 300 m in thickness (Tokarski, 1989), the rock association with dark gray to black colors is composed of interlayered sandstone, argillite, and thickly bedded and unstratified sandstone. Within the argillite-quartz wacke section are 1-meter-thick units of thinly laminated, alternating argillite and quartz wacke in which the laminae average 25 mm in thickness (Figures 2, 3a). And the formation is locally transected by fractures and sandstone dikes up to several centimeters wide and is intruded by pyrophyllite dikes (Figure 3b). Moreover, metasedimentary rocks on the Schmidt Peninsula are interpreted to be located on a steeply north-dipping limb of a major anticline, which plunges to the west-northwest at ~37° (Tokarski, 1989; Halpern, 1965). The O'Higgins Fault is a major structure in the region, nevertheless, the amount and direction of displacement along the fault are unclear (Halpern, 1965). However, activity along this fault was likely related to the northeastward subduction of the Pacific Plate and associated plates beneath the Antarctic Plate.

Figure  2.  Composite stratigraphic section at Schmidt Peninsula, Antarctica (after Halpern, 1965).

DownLoad: Full-Size Img PowerPoint

Figure  3.  The field photograph ((a), (b)) and micrograph features of sediments in Schmidt Peninsula ((c), (d), (e), (f)).

DownLoad: Full-Size Img PowerPoint

The samples collected from Legoupil Formation on the Schmidt Peninsula are dark gray to black (Figure 3) and comprise graywacke (SP1, SP3, SP5, SP6, and SP10), lithic arenite (SP9 and SP4). Samples SP3, SP5, and SP6 are locally folded with millimeter- to meter-scale fold amplitudes (Figure 3c). Quartz veins (0.5–10 cm wide) transect the formation and are oriented both parallel and oblique to the schistosity (Figure 3a). Samples SP4 and SP10 contain thick (up to several centimeters) sandstone intercalations.

2. ANALYTICAL METHODS

2.1 Zircon U-Pb Dating

In order to get the geochronological information of Legoupil Formation, SP5 and SP9 were chosen for selecting zircons. Preparation and imaging of zircon grains were conducted at the Langfang Geophysical Survey, Langfang, Hebei Province, China. Zircon grains were separated from crushed samples using conventional methods (sieving, magnetic separation, heavy liquid separation, and hand picking). Avoiding missing the age information, zircons were randomly selected and were then mounted on an epoxy resin disc and polished to expose internal growth zones. Transmitted and reflected light images were obtained using an optical microscope and cathodoluminescence (CL) images were collected with a JEOL scanning electron microscope in order to reveal internal structures and to select the dating points.

Zircon grains were dated using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) at the Chinese Academy of Geological Sciences, Beijing, in China. We used a Finnigan Neptune MC-ICP-MS coupled to a Newwave UP213 laser ablation system and followed the procedures described by Hou et al. (2009). ICPMSDataCal software was used to perform offline data, including quantitative corrections of U-Pb ages. Furthermore, the data were reduced using the IsoplotR (Vermeesch, 2018) and common Pb was corrected following Anderson (2002). Uncertainties in the measured isotopic ratios are reported at the 1σ level.

2.2 WHOLE-ROCK ELEMENTAL ANALYSIS

The geochemcial compositions of the samples were measured at the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences. Major elements were measured on fused glass disks using a PW4400 X-ray fluorescence spectrometer. Trace elements were analyzed by ICP-MS with a PE300D instrument, following acid digestion of samples in Teflon bombs. respectively. The test methods followed those in GB/T 14506.28-2010 and GB/T 14506.30-2010 and the samples of BHVO-1, AGV-1 and G-2 are used for testing accuracy. The precision and accuracy are better than 5% for major elements and 10% for major and trace elements, respectively.

3. ANALYTICAL RESULTS

3.1 Petrography

The Legoupil Formation samples comprise predominantly wacke and lithic arenite (Figures 2, 3). Lithic arenite are composed of detrital materials (96%) and argillaceous materials (4%), while the detrital materials are composed of quartz (51%), feldspar (34%) and rock fragments (15%). However, the rock fragments are mainly Quartz minerals. The wacke samples contain grains that are poorly sorted and angular, and are therefore classified as texturally immature micro-breccias. The arenite samples contain grains that are less than 0.3 mm and moderately sorted and poorly rounded. Samples SP9 and SP4 comprise of mudstone rock fragments (< 10%), and have sub-mature compositions. Discordant quartz veins occur within the Legoupil Formation (Figure 3d), and silica was likely mobilized due to increase in pressure and temperature associated with folding and faulting (Harker, 1950). Silica in the veins was likely remobilized from fine clastic quartz and silicate minerals in quartz-rich arenaceous and argillaceous sediments of the Legoupil Formation.

3.2 Detrital Zircon U-Pb Geochronology

Zircon U-Pb ages for the analyzed samples are listed in Table S1, Figures 4, 5 and are discussed below in detail.

Figure  4.  The CL images of zircons and cumulative frequency distribution plots for SP5 and SP9 in Schmidt Peninsula.

DownLoad: Full-Size Img PowerPoint

Figure  5.  Zircon U-Pb concordia diagrams and dating results of Sample S5 (a), (b), (c) and S9 (d), (e), (f).

DownLoad: Full-Size Img PowerPoint

3.2.1   Sample SP5

zircon grains from sample SP5 are subhedral or fragmented with 50–120 μm, preserve fine-scale oscillatory growth zoning (Figure 4a), and yield Th/U ratios of 0.15–1.58 (Table S1), indicative of an igneous origin (Pupin, 1980). Seventy analyzed grains yield U-Pb ages of 1 992–265 Ma, with peaks at 271 and 470 Ma (Figure 4b). Most of the zircon ages are concordant (Figure 5a), and thirty zircon grains have the Permian U-Pb ages from 265 to 295 Ma, two zircon grains have the Carboniferous U-Pb ages from 327 to 339 Ma, one zircon grain has the Devonian age of 384 Ma, twenty zircon grains have the Ordovician U-Pb ages from 452 to 484 Ma, three zircon grains have the Cambrian ages from 510 to 523 Ma, and fourteen zircon grains have the Proterozoic ages from 570 to 1 992 Ma.

3.2.2   SAMPLE SP9

Zircon grains with 50–100 μm from sample SP9 are similar in appearance to those from sample SP5 (Figure 4c) and yield Th/U ratios of 0.18–1.79 (Table S1), also indicative of an igneous origin. Seventy zircon analyses yield ages of 1 732–262 Ma, with peaks at 271 and 475 Ma (Figure 4d). Most of the zircon ages are concordant (Figure 5d), and thirty-seven zircon grains have the Permian U-Pb ages from 262 to 291 Ma, five zircon grains have the Carboniferous U-Pb ages from 302 to 351 Ma, five zircon grains have the Devonian ages from 370 to 415 Ma, two zircon grains have the Silurian ages from 431 to 436 Ma, twelve zircon grains have the Ordovician U-Pb ages from 450 to 486 Ma, three zircon grains have the Cambrian ages from 504 to 527 Ma, and six zircon grains have the Proterozoic ages from 745 to 1 732 Ma.

3.3 Geochemistry

The geochemical compositional data of seven samples are given in Table 1.

Table  1.  Composition of major, trace and rare earth elements of the Legoupil Formation sediments

Element	SP1		SP3		SP4		SP5		SP6		SP9		SP10	Element	SP1		SP3		SP4		SP5		SP6		SP9		SP10
	Gray wacke		Gray wacke		Lithic arenite		Gray wacke		Gray wacke		Lithic arenite		Gray wacke		Gray wacke		Gray wacke		Lithic arenite		Gray wacke		Gray wacke		Lithic arenite		Gray wacke
SiO2	66.79		64.26		66.83		66.6		64.46		69.36		58.81	Zr	150		162		119		173		167		151		127
Al2O3	15.41		15.74		13.82		16.4		16.26		14.23		18.29	Hf	4.56		4.99		3.64		5.09		5.37		4.35		4.04
CaO	1.86		2.37		1.86		0.9		1.64		1.59		1.09	La	29.3		33.4		33.9		35.8		36		21.1		37.8
Fe2O3	1.97		1.96		2.36		0.95		1.77		0.77		1.86	Ce	62		72.8		72.5		75.9		76.9		42.9		82.5
FeO	2.3		3.41		3.24		2.95		3.27		2.48		4.92	Pr	7.45		8.84		8.54		9.02		9.34		5.29		10
K2O	3.18		3.61		3.52		2.54		3.97		2.19		4.24	Nd	27.7		33.1		30.5		32.6		35		20.1		38.1
MgO	1.85		2.22		2.01		1.88		2.15		1.48		2.99	Sm	5.38		6.56		6.19		6.06		7.09		3.72		7.49
MnO	0.06		0.08		0.06		0.06		0.07		0.06		0.08	Eu	1.21		1.41		1.26		1		1.39		0.83		1.45
Na2O	2.99		1.94		1.11		4.75		2		4.43		1.96	Gd	4.85		6.1		5.6		4.91		6.31		3.13		6.55
P2O5	0.13		0.17		0.42		0.14		0.19		0.11		0.17	Tb	0.75		0.96		0.88		0.77		1.03		0.5		1.05
TiO2	0.54		0.7		0.69		0.61		0.69		0.41		0.9	Dy	4.14		5.49		4.99		4.3		5.86		2.68		5.83
CO2	0.3		0.21		0.52		0.2		0.3		0.61		0.3	Ho	0.8		1.05		0.94		0.77		1.09		0.5		1.09
H2O+	1.94		2.74		3.09		2		2.54		1.68		3.78	Er	2.38		3.09		2.83		2.28		3.29		1.5		3.26
LOI	2.18		2.62		3.68		2.13		2.65		2.22		3.7	Tm	0.36		0.48		0.45		0.36		0.51		0.23		0.51
Rb	146		156		147		111		172		76.5		204	Yb	2.48		3.17		2.85		2.35		3.34		1.56		3.27
Sr	259		341		234		63.2		167		402		207	Lu	0.37		0.47		0.43		0.35		0.49		0.23		0.48
Cs	6.21		6.8		7.08		5.34		8.22		2.82		10	Sc	11.3		13.7		13.2		10.7		13.1		7.81		18.1
Ba	616		729		560		427		658		428		687	Y	23.8		29.5		26.2		21.3		29.5		15.1		30.9
Pb	16.2		17.2		49.5		15.7		16.7		20.2		27.9	Ti	3 279		4 048		3 981		3 600		3 904		2 377		5 375
Bi	0.28		0.25		0.47		0.22		0.38		0.13		0.49	Co	8.73		12.0		9.30		10.5		8.05		8.98		18.2
Th	10.7		12.5		11.9		11.6		12.9		6.86		14.2	La/Sc	2.59		2.44		2.57		3.35		2.75		2.70		2.09
U	2.63		3		2.61		2.71		3.38		2.09		3.76	Ti/Zr	21.86		24.99		33.45		20.81		23.38		15.74		42.32
Nb	10.5		13.6		13.6		12.2		13.5		7.16		16.3	Co/Th	0.82		0.96		0.78		0.91		0.62		1.31		1.28
Ta	0.8		0.98		0.96		0.9		1.05		0.54		1.19	La/Th	2.74		2.67		2.85		3.09		2.79		3.08		2.66

 | Show Table

DownLoad: CSV

3.3.1   Major element compositions

The samples contain 58.81 wt.%–69.36 wt.% SiO₂, similar to typical Paleozoic wacke. They yield Al₂O₃ contents of 13.82 wt.%–18.29 wt.%, indicating the presence of mica or clay minerals, and low CaO (0.9 wt.%–2.37 wt.%) and MgO (1.48 wt.%–2.99 wt.%) contents, indicating a low proportion of carbonatite. Low MnO (0.06 wt.%–0.08 wt.%) and P₂O₅ (0.11 wt.%–0.42 wt.%) contents indicate the presence of minor apatite. The samples also contain 2.3 wt.%–4.92 wt.% FeO, 0.77 wt.%–2.36 wt.% Fe₂O₃, 2.19 wt.%–4.24 wt.% K₂O, and 1.11 wt.%–4.75 wt.% Na₂O.

The Legoupil Formation samples are classified as wacke in the geochemical classification diagram of Herron (1988), except for two samples (SP4 and SP9) that are plotted in the lithic arenite field (Figure 6a). Similar classifications are made when the samples are plotted on a log(SiO₂)/(Al₂O₃) vs. log(Na₂O)/(K₂O) diagram (Figure 6b). Major element compositions can be used to classify sedimentary rocks and determine compositional maturity. The samples yield SiO₂/Al₂O₃ and Al₂O₃/TiO₂ ratios of 3.21–4.87 (mean = 4.19) and 20.03–34.71, respectively, indicating a moderate degree of weathering (Roser et al., 1996). Our major element data, together with the petrographic observations, indicate that the Legoupil Formation is compositionally immature compared with average shale (Taylor and Mclennan, 1985).

Figure  6.  Major element classification of the Legoupil Formation sandstones. (a) Modified after reference (Herron, 1988); (b) modified after reference (Pettijohn, 1972).

DownLoad: Full-Size Img PowerPoint

3.3.2   Trace element compositions

The samples yield total rare earth element (REE) contents (∑REE) of 119.37 ppm–230.28 ppm, which is similar to the composition of the UCC (∑REE = 148.14 ppm) and PASS (∑REE = 184.77 ppm) reservoirs (Mclennan, 2001; Taylor and Mclennan, 1985). Chondrite-normalized REE diagrams display clear negative Eu anomalies with an average δEu of 0.66 (Figure 7, Table 1). The samples yield average La_N/Yb_N ratios of 8.22, indicating enrichment of light REE (LREE), and contain average contents of 32.47 ppm La, 69.36 ppm Ce, 31.01 ppm Nb, 12.56 ppm Sc, 149.86 ppm Zr, 11.52 ppm Th, 12.41 ppm Nb, 10.82 ppm Co, 77.57 ppm Zn, and 95.2 ppm V. They yield average La/Y, La/Th, Zr/Th and La/Sc ratios of 1.31, 2.84, 13.68 and 2.64, respectively.

Figure  7.  Chondrite-normalized REE compositions (normalization values after Boynton (1984)) for the Legoupil Formation sandstones. The data in color area are from Castillo et al. (2016).

DownLoad: Full-Size Img PowerPoint

4. DISCUSSION

4.1 Timing of Legoupil Formation Deposition

Conventionally, the Legoupil Formation is recognized as a significant part of the Trinity Peninsula Group. A Permian–Triassic maximum depositional age has been assigned to most parts of the Trinity Peninsula Group based on detrital zircon U-Pb ages (Castillo et al., 2016, 2015). However, Hornblende K-Ar dating of a diorite pebble from a pebbly mudstone unit (116 Ma) and the age of an andesite that is unconformably overlying the Legoupil Formation (74.7 Ma) indicate that the Legoupil Formation formed in the Cretaceous and is younger than the Trinity Peninsula Group (Halpern, 1965). Thus, detrital zircons show different information, zircons of the Legoupil Formation yield a dominant population of ages at 339–276 Ma (Barbeau et al., 2010) with a significant peak at 271 Ma (Castillo et al., 2016). Furthermore, Early Triassic bivalve fossils in Legoupil Formation have also been identified on Kopaitic and Gandara islands (Thomson, 1975) while Tokarski (1989) concluded that the bivalves appear to have been redeposited.

The age distribution of Legoupil Formation detrital zircon grains includes representative Paleozoic and Proterozoic ages. Therefore, the samples host a major population of Permian igneous (Figure 4) and share a secondary Ordovician component of ca. 470 Ma, consistent with previous studies (Castillo et al., 2016; Fanning et al., 2011; Barbeau et al., 2010). In general, the youngest grains in a detrital zircon population are a particularly valuable approach when determining the age of non-fossil-bearing sedimentary rocks and can be used to constrain the maximum depositional age of a sedimentary rock (Dickinson and Gehrels, 2009). Based on the youngest concordant zircon ²⁰⁶Pb/²³⁸U age (262 ± 3 Ma), we can assume the maximum depositional age of Legoupil Formation is younger than Late Permian with fossil evidence suggesting that Legoupil Formation is Triassic.

4.2 Sedimentary Provenance

Elemental ratios in sedimentary rocks can constrain material cycling and source weathering, while the maturity of sediments can be determined by analyzing SiO₂/Al₂O₃ and La/Sc ratios which record the abundance of amphibole, pyroxene, zircon and apatite (Roser and Korsch, 1986).Moreover, the remarkably similar patterns of peaks and troughs between sedimentary and igneous ages suggest that the sedimentary record is a valid representation of the magmatic record (Condie, 1993). Consequently, the U-Pb isotopic composition of detrital zircon grains can be used to constrain the source of sedimentary rocks.

4.2.1   Geochemical provenance indictors

Petrological study of the Trinity Peninsula Group suggests that the Legoupil Formation was deposited in a distal basin or represents the recycling of older sedimentary rocks (Castillo et al., 2015). Furthermore, their geochemistry indicates that Trinity Peninsula Group sediments were sourced mainly from a felsic igneous source, typical of rocks derived from silicic crystalline (plutonic-metamorphic) terrains containing intermediate-acid volcanic components (Castillo et al., 2015).

Our petrographic observations allow the Legoupil Formation sandstones to be classified as wacke and lithic arenite (Figure 6). The constituent grains (quartz, feldspar, and mudstone debris) are sub-rounded, indicating that the Legoupil Formation sediments are immature, as supported by a relatively high proportion of feldspar. The grain-size distribution of sandstones is controlled mainly by transport processes. The Legoupil Formation lithic arenite is poorly sorted and is therefore interpreted as sediment that was rapidly weathered and deposited with relatively little winnowing (Dott, 1965) and the existence of mudstone debris represents the partial recycling of older sedimentary rocks.

Moreover, the Legoupil Formation samples yield low SiO₂ contents, and are consistent with their low quartz contents indicating a relatively low maturity. The Legoupil Formation samples yield average Al₂O₃/TiO₂ ratios of ~25.22, similar to those of felsic igneous rocks (Hayashi et al., 1997), while incompatible to compatible element ratios also indicate a felsic provenance (Figure 8). However, the geochemical features show the rocks of Legoupil Formation should be from a primary felsic source similar to the upper continental crust composition (Condie, 1993).

Figure  8.  Provenance discriminant diagrams for the sandstones of Legoupil Formation. (a) Modified after Floyd and Leveridge (1987); (b) modified after Shen et al. (2009); (c) modified after Yang et al. (2017); (d) modified after Roser and Korsch (1988). Average compositions of crust on plot Figure 8a are from Floyd and Leveridge (1987), average compositions of rocks on plot Figure 8c are from Condie (1993). The data in grey area are from Castillo et al. (2016). Discrimination functions in Figure 8d are F₁ = -1.773 × TiO₂ + 0.607 × Al₂O₃ + 0.76 × Fe₂O₃^T - 1.5 × MgO + 0.616 × CaO + 0.509 × Na₂O - 1.224 × K₂O - 9.09; F₂ = 0.445 × TiO₂ + 0.07 × Al₂O₃ - 0.25 × Fe₂O₃^T - 1.142 × MgO + 0.438 × CaO + 1.475 × Na₂O + 1.426 × K₂O - 6.861. Fields in Figure 8d are: P₁. mafic provenance; P₂. intermediate provenance; P₃. felsic provenance and P₄. quartzose recycled provenance. TPG. Trinity Peninsula Group.

DownLoad: Full-Size Img PowerPoint

Furthermore, the uniform SiO₂/Al₂O₃ (3.22–4.87) and La/Sc (0.25–0.46) ratios further indicate a moderate maturity formation deposited in a tectonically active setting (Cox et al., 1995). And the ratios of La/Sc (2.09–3.35), Ti/Zr (15.74–42.32), Co/Th (0.62–1.31) and La/Th (0.62–1.31) are also consistent with a source in the upper crust of a continental island arc while the La/Th ratios reflect the influence of a magmatic arc in the hinterland (Mclennan et al., 1993; Bhatia, 1985).

4.2.2   Detrital zircon provenance indicators

Detrital zircons are an effective tool to analyze the sedimentary provenance and the crustal evolution (Cawood et al., 2012; Hawkesworth et al., 2010). To determine the provenance of the Legoupil Formation and Trinity Peninsula Group, 60 detrital zircon samples were analyzed from the Ellswort Whitmore Mountains (EWM), Transantarctic Mountains (TM), Victoria Land, Duque de Youk Complex, and Trinity Peninsula Group, including those acquired in the present study. Zircon grains from the Legoupil Formation are composed of Permian, Ordovician and a minor population of Proterozoic–Carboniferous grains with dominant signature peaks of ~270 and ~470 Ma subpopulations. The Permian zircon grains are subhedral or fragmented and display fine-scale oscillatory growth zoning. They likely record a proximal magmatic event, possibly related to arc magmatism, and indicate that transport from the source region was limited. Similar geochronological results suggest that the Trinity Peninsula Group shares its provenance with the Duque de Youk Complex (Figure 9), therefore the Choiyoi igneous province might represent the primary source for both units (Fanning et al., 2011). However, Permian granites also occur in the Antarctic Peninsula (Millar et al., 2002), Marie Byrd Land (Pankhurst et al., 2006), the Ellsworth Mountains (Flowerdew et al., 2007) and Transantarctic Mountains (Elliot et al., 2015) (Figure 10), which could also represent the source of Permian zircon grains in the Legoupil Formation. Ordovician ages are the second most common in the Legoupil Formation detrital zircon samples. This age has also been reported from the Eden Glacier area (Riley et al., 2012) and View Point Formation (Bradshaw et al., 2012; Millar et al., 2002) in the Antarctic Peninsula (an Ordovician protolith age and conglomerate clast), the northern North Patagonian Massif (Pankhurst et al., 2014, 2006), the Famatinian magmatic arc in northwestern Argentina (Pankhurst et al., 1998), and the Cape fold belt in South Africa (Craddock et al., 2017). These observations indicate that the Ordovician detrital zircon grains in the present study were sourced from the Antarctic Peninsula and South America. The analyzed zircon grains yield only a minor population of Neoproterozoic–Carboniferous ages. However, Neoproterozoic–Cambrian detrital zircon grains have recently been reported in the region (Blewett et al., 2019; Elliot et al., 2015; Paulsen et al., 2015; Goodge et al., 2010), which reveal the presence of old basement beneath the present-day icecap (Craddock et al., 2017). Zircon grains from the Legoupil Formation with ages of ca. 550 Ma yield highly negative initial ɛ_Hf values (Bradshaw et al., 2012), similar to zircons from the Ellsworth Mountains in West Antarctica (Flowerdew et al., 2007). Furthermore, Devonian–Carboniferous magmatism is recorded at Target Hill (Riley et al., 2012) and the Patagonian Plateau in South America (Pankhurst et al., 2003).

Figure  9.  The Kernal Density Estimate of U-Pb ages for detrital zircon grains from Trinity Peninsula Group and the samples location of Trinity Peninsula Group (the data are from this study and Castillo et al. (2017, 2016, 2015).

DownLoad: Full-Size Img PowerPoint

Figure  10.  The location of Antarctic Peninsula within the southwestern Gondwana margin in Permian to Early Triassic (Elliot et al., 2015).

DownLoad: Full-Size Img PowerPoint

In summary, there are strong age-distribution similarities of pre-Permian metasedimentary and igneous rocks and O and Hf isotopic characteristics for Permian detrital zircons and igneous and magmatic zircons between Antarctic Peninsula and southern Patagonia (Castillo et al., 2017; Barbeau et al., 2010), we predict that the Legoupil Formation sediments are likely to be from eastern domain of Antarctic Peninsula and the southern tip of Patagonia.

4.3 Geological Significance

It has generally been accepted that the Permian volcano-plutonic arc was likely located along the active Gondwana continental margin, probably in the south Patagonia-Antarctic Peninsula sector. A detailed study of the Permian detritus can provide further clues about the nature and evolution of the Permian magmatism along the southern margin of West Gondwana (Castillo et al., 2016).

Geochemical compositions can be used to determine the tectonic setting of sedimentary basins (Table 2). Trace elements such as Th, La, and Sc are stable during sediment transportation and diagenesis, and are therefore reliable indicators of tectonic setting. The major element (e.g., SiO₂, Fe₂O₃, TiO₂, and P₂O₅) and trace element (e.g., Co, and Ti) concentrations of the studied samples are similar to those of continental island arcs (Bhatia and Crook, 1986). Furthermore, the La/Sc, Ti/Zr, Co/Th, and La/Th ratios of the Legoupil Formation samples (2.09–3.35, 15.74–42.32, 0.62–1.31, and 0.62–1.31, respectively) are also similar to those of continental island arc. The Legoupil Formation samples also plot in the continental island arc and active continental margin fields of the classification diagram of Bhatia (1985) in Figures 11, 12. We therefore conclude that the Legoupil Formation formed in a continental island arc setting and active continental margin.

Table  2.  Trace element characteristics of graywackes from various tectonic settings

	Schmidt		OIA			CIA			ACM			PCM
	Average		X	±L		X	±L		X	±L		X	±L
SiO2	65.03		58.83	1.6		70.69	2.6		73.86	4		81.95	6.2
Al2O3	15.73		17.11	1.7		14.04	1.1		12.89	2.1		8.41	2.2
Fe2O3	1.66		1.95	0.5		1.43	0.5		1.3	0.5		1.32	1.6
FeO	3.22		5.52	2.1		3.05	0.4		1.58	0.9		1.76	1.2
CaO	1.61		5.83	1.3		2.68	0.9		2.48	1		1.89	2.3
MgO	2.08		3.65	0.7		1.97	0.5		1.23	0.5		1.39	0.8
Na2O	2.74		4.1	0.8		3.12	0.4		2.77	0.7		1.07	0.6
K2O	3.32		1.6	0.6		1.89	0.5		2.9	0.5		1.71	0.6
TiO2	0.65		1.06	0.2		0.64	0.1		0.46	0.1		0.49	0.2
P2O5	0.19		0.26	0.1		0.16	0.1		0.09			0.12
MnO	0.067		0.15			0.1			0.1			0.05
Fe2O3*+MgO	7.33		11.73			6.79			4.63			2.89
Al2O3/SiO2	0.24		0.29			0.2			0.18			0.1
K2O/Na2O			0.39			0.61			0.99			1.6
Al2O3/(Na2O+CaO)	3.88					2.42			2.56			4.15
Pb	23.34		6.9	1.4		15.1	1.1		24	1.1		16	3.4
Th	11.52		2.27	0.7		11.1	1.1		18.8	3		16.7	3.5
Zr	149.86		96	20		229	27		179	33		298	80
Hf	4.58		2.1	0.6		6.3	2		6.8	-		10.1	-
La	32.47		8.72	2.5		24.4	2.3		33	4.5		33.5	5.8
Ce	69.36		22.53	5.9		50.5	4.3		72.7	9.8		71.9	11.5
Nd	31.01		11.36	2.9		20.8	1.6		25.4	3.4		29	5.03
Nb	12.41		2	0.4		8.5	0.8		10.7	1.4		7.9	1.9
Ti	0.38		0.48	0.12		0.39	0.06		0.26	0.02		0.22	0.06
Sc	12.56		19.5	5.2		14.8	1.7		8	1.1		6	1.4
V	95.23		131	40		89	13.7		48	5.9		31	9.9
Zn	77.57		89	18.6		74	9.8		52	8.6		26	12
Co	10.82		18	6.3		12	2.7		10	1.7		5	2.4
La/Y	1.31		0.48	0.12		1.02	0.07		1.33	0.09		1.31	0.26
La/Th	2.84		4.26	1.2		2.36	0.3		1.77	0.1		2.2	0.47
La/Sc	2.64		0.55	0.22		1.82	0.3		4.55	0.8		6.25	1.35
Zr/Th	13.68		48	13.4		21.5	2.4		9.5	0.7		19.1	5.8

 | Show Table

DownLoad: CSV

Figure  11.  Bivariate plot of K₂O/Na₂O vs. SiO₂ (Roser and Korsch, 1986), bivariate plot of Al₂O₃/SiO₂ vs. (Fe₂O₃^T + MgO) and bivariate plot of TiO₂ vs. (Fe₂O₃^T+MgO) (Bhatia, 1985) for discrimination of the tectonic settings of the Legoupil Formation sandstones. PM. Passive margin; ACM. active continental margin; CIA. continental island arc; OIA. oceanic island arc. The data in grey area are from Castillo et al. (2016).

DownLoad: Full-Size Img PowerPoint

Figure  12.  Trace element plots of sandstones from Legoupil Formation in Schmidt Peninsula for tectonic discrimination (Bhatia and Crook, 1986). PM. Passive margin; ACM. active continental margin; CIA. continental island arc; OIA. oceanic island arc. The data in grey area are from Castillo et al. (2016).

DownLoad: Full-Size Img PowerPoint

Moreover, the U-Pb zircon age distribution in Legoupil Formation shows a major input of Permian Age grains, which is similar to the convergent margin basins that have a high proportion of detrital zircons with ages close to the age of the sediment (Cawood et al., 2012). In order to know the depositional basin setting, we accepted the minimum depositional age of Legoupil Formation similar to the age of Trinity Peninsula Group that is assumed to be formed during Guadalupian to Early Triassic (Castillo et al., 2016).

The age spectrum from Legoupil Formation displays the similar age patterns of sediments deposited in a convergent setting (Cawood et al., 2012). While the detrital zircon age data of Legoupil Formation plotted with minimum deposited ages (240 Ma) have been illustrated in Figure 13 and also reflect that the Legoupil Formation is deposited in a convergent setting (Figure 13). Together with the previous studies, geochemistry and detrital zircons reflect an active continental margin tectonic setting. Furthermore, convergent margin basins have a high proportion of detrital zircons (generally greater than 50%) with ages close to the age of the sediment (Cawood et al., 2012) and the detrital zircon spectra of Legoupil Formation are similar to the ones deposited in trench basins (Figure 13b) (Hyden and Tanner, 1981).

Figure  13.  Variation of the differences between the crystallization ages of detrital zircons (a) and the depositional age of the sedimentary sequence, plotted as cumulative proportion curves (b), modified after Hawkesworth et al. (1981). A. Convergent setting; B. collisional setting; C. extensional setting.

DownLoad: Full-Size Img PowerPoint

5. CONCLUSIONS

(1) The Legoupil Formation on the Schmidt Peninsula comprises wacke and lithic arenite, and is compositionally and texturally immature.

(2) The Legoupil Formation has a strong and narrow peak of 270 Ma and subpopulation peak of 470 Ma, consistent with the previous studies. The youngest zircon age of 262 Ma could represents the maximum age for the Legoupil Formation and the Legoupil Formation is likely to be deposited in the Early Triassic with the fossil evidence.

(3) Geochemical compositions indicate that the Legoupil Formation was derived from a felsic source and was likely to be from eastern domain of Antarctic Peninsula and the southern tip of Patagonia based on the O and Hf isotopic features.

(4) The age spectrum from Legoupil Formation displays the similar age patterns of sediments deposited in a convergent setting. Together with the geochemical features, Legoupil Formation could be deposited in a forearc tectonic setting under active continental margin.