Use of Detrital-zircon U–Pb age-distribution Comparison to Infer Source Location

Provenance analysis for volcanism without eld evidence remains a major challenge. Detrital zircon grains from 13 samples of the Middle–Upper Triassic Xiaoquangou Group in the Southern Junggar Basin (SJB) were analyzed using U–Pb geochronology to constrain the location and characteristics of Triassic volcanism in the area as well as to understand its tectonic implications. A comparison of the distribution of detrital zircon U–Pb ages reveals Triassic zircon ages predominate in northern Bogda Mountains, with subordinate contributions also in southern Bogda Mountains, and no or minimal input in North Tianshan piedmont. The geochronology data combined with the euhedral and angular zircon grains suggest that the Triassic zircons probably originate from Bogda Mountains. A comparative provenance analysis reveals varied sources for Xiaoquangou Group in the SJB, with sediments of the Bogda Mountains area derived mainly from North Tianshan, Central Tianshan, and Bogda Mountains. The supply of sediments from Bogda Mountains started in the Late Triassic, and is indicative of the initial uplift of Bogda Mountains. This study proves the effectiveness of the comparison of detrital zircon U–Pb age distributions for inferring source characteristics and is applicable in similar situations, particularly when the source area is poorly preserved.


Introduction
Detrital zircon U-Pb geochronology is widely exploited for source-to-sink analysis (e.g. Cawood Fig. 1, the detrital zircon U-Pb age pattern reveals an age group "D", with no related magmatic rocks in potential source areas. This situation is common for deep-time source-to-sink analysis due to intense denudation in source areas and poor exposure of stratigraphic record (Sømme, Wang et al. 2018bWang et al. , 2019b. The presently exposed geology of potential source regions does not include Triassic age igneous rocks and therefore there is no way to correlate the existence of such grains in sandstones with a source area. These Triassic zircons, nevertheless, has been interpreted as derivation from the Jiumusaer area of the Bogda Mountains (BGMRXUAR, 1993), Central Tianshan (Yin et al. 2018), and Harlik Mountains (Ji et al. 2018).
In this contribution, we assess the provenance of the Middle-Upper Triassic Xiaoquangou Group in southern Junggar Basin (SJB) through a comparison of the detrital zircon U-Pb age distributions of thirteen samples which collected from different sink areas. These areas included the Sikeshu, Jiangongmeikuang, Aiweiergou, Haojiagou, Jingjingzigou, Haxionggou, Baiyanghe, Xidalongkou, Dongdalongkou, Taodonggou, Dabancheng and Yuergou sections, and Well C regions (12 sections and 1 well in total). The emphasis of this study is to explore an approach to infer the source location in the absence of direct eld evidence for related detrital-zircon U-Pb ages of the potential source area. It also utilizes other information (e.g. trace elements contents) from the detrital zircons to further characterize the source features.

Geological Setting
The Junggar Basin is a triangular-

Sampling And Analytical Methods
Medium-to coarse-grained sandstones were collected from eight sections of the Middle-Upper Triassic Xiaoquangou Group in SJB for detrital zircon U-Pb analysis (Fig. 2b). The samples including 12SKS-07,

Zircon trace elements
Results from geochemical analysis of the detrital zircons are listed in Supplemental Table 2. The zircons show a wide range of rare earth elements (REEs) concentrations (218-10178 ppm, mostly 400-900 ppm), with the average concentrations signi cantly above that for chondrite (2.65 ppm; Sun and McDonough, 1989  Moreover, some other evidences support that the Triassic zircons are probably source from the Bogda Mountains. (1) Paleocurrent observations show complicated drainage system with both northward-and southward-directed paleocurrents (Fig. 8). The paleocurrents are principally perpendicular to the present day Bogda Mountains in the western Bogda Mountains, while in the eastern Bogda Mountains the paleocurrents mainly northward-directed (Fig. 8). This proved that the western Bogda Mountains probably have uplifted and began to provide sources. Instead, the eastern Bogda Mountains (e.g., Zaobishan region) remain accept as a sink region with the sources from NTS and CTS.  (Fig. 9). This inference is further supported by the REEs abundance and chondritenormalized REE patterns (Fig. 6). The Triassic zircons show close REEs concentrations and chondritenormalized REE patterns to the Cambrian-Permian zircons, suggesting they were derived from similar source rock types ( Samples XJ12-06, 16TDG-06, and 17DDLK-09 (group 2) from the Aiweiergou, Taodonggou and Dongdalong sections are assigned similar sediment sources owing to their close links in the MDS plot (Fig. 10). The Mississippian and the Cambrian-Devonian magmatic rocks from the CTS are likely major sources for group 2, considering the relation of their spectra (Fig. 5). Group 2 also displays minor Pennsylvanian zircon grains in the detrital zircon U-Pb age probability plot that are likely related to the NTS sources.
Samples 12SKS-07, 17JGMK-06, and 16DBC-04 (group 3) from the Sikeshu, Jiangongmeikuang, and Dabancheng sections generally display a limited age variation dominated by the Pennsylvanian (Fig. 5), and show proximity in the MDS plot (Fig. 10). The Pennsylvanian age distribution combined with the compositionally and texturally immature detrital grains, probably originate from the magmatic rocks of NTS with zircon U-Pb ages of 320-300 Ma (Fig. 5). Furthermore, a little Cambrian-Devonian and Mississippian age groups in the probability plot also suggest contribution to group 3 from the CTS.
Particularly, the probability plot of detrital zircon ages for the Dabancheng section (sample 16DBC-04) shows an age group spanning 250-200 Ma, re ecting further contribution from the Bogda Mountains to the Dabancheng section.
Samples 15HXG-95, 17JJZG-10, 17BYH-61, and HX-01 (group 4) from the Haxionggou, Jingjingzigou, Baiyanghe and Yuergou sections show a mixture age variation of Pennsylvanian (320-300 Ma) and Mississippian (360-320 Ma), and have proximity in the MDS plot (Fig. 10). The two major peaks with age group spanning 320-300 Ma and 360-320 Ma, revealing the major provenance of group 4 is from NTS and CTS. Besides, the presence of some Triassic zircons in sample HX-01 from the Yuergou section suggests possible provenance from the Bogda Mountains.
The systematic analysis of the detrital zircons preserved in the Xiaoquangou Group allowed us to discriminate their source areas and establish the source-to-sink system in the Middle-Late Triassic for SJB. The provenance analysis of thirteen samples partitions SJB and adjacent areas into four groups (Fig. 11), which is consistent with groups from the MDS plot (Fig. 10). The Bogda Mountains, CTS, and NTS appear to be the dominant sources for group 1, group 2, and group 3, respectively (Fig. 10). The mixture of NTS and CTS appear to be the dominant sources for group 4 (Fig. 10). Although the source areas of SJB include the Bogda Mountains, NTS, and CTS in the Middle-Late Triassic, the western (NTS areas) and eastern (Bogda Mountains areas) parts of SJB exhibit inconsistencies in provenance patterns. In the Bogda Mountains, the provenance is attributed to the Bogda Mountains itself, NTS and CTS (Figs. 10 and 11). Of these, the Bogda sources mainly supply near regions, such as Haojiagou section, Xidalongkou section and Well C. In contrast, the NTS piedmont (the Sikeshu and Jiangongmeikuang sections) received detritus mainly from the NTS, with minor input from the CTS and no from the Bogda Mountains. In conclusion, the NTS accounts primarily for provenance of the western part of SJB (NTS areas), while sediments of the eastern part of SJB (Bogda Mountains areas) are principally from the Bogda Mountains and/or CTS (Figs. 3, 5, 10, 11).

Implication for the initial uplift of the Bogda Mountains
The Middle-Upper Triassic Xiaoquangou Group in the Well C, Haojiagou and Xidalongkou sections yields a complex age population with many syndepositional Triassic zircons from the Bogda Mountains (Fig. 5), indicating the initiation of sediment supply from the Bogda Mountains in the same area. We attribute this abrupt change to the initial uplift of the Bogda Mountains (referred as uplift stage; Wang et al. 2019b). Moreover, the Xiaoquangou Group in the Haojiagou and Xidalongkou sections contain lithic fragments (Wang et al. 2018b(Wang et al. , 2019b, with the sediments characterized by moderate chemical weathering and sedimentary recycling (Wang et al. 2019a(Wang et al. , 2019b. This suggests that the Bogda Mountains provided syndepositional magmatic rocks and recycled siliciclastics to the basin, probably pointing to a "sinksource-sink" process. In the cumulative frequency curves of the lag time (lag time = crystallization agedeposition age; Fig. 12), the sediments of the uplift stage show a near "bimodal lag time" including low and high lag time groups compared to mainly low lag time groups for the syn-rift and post-rift sediments (Fig. 12). The high lag time sediments probably arise from sedimentary recycling, while the low lag time sediments are attributed to the syndepositional magmatic activity. In addition, although the proportions of the Cambrian-Devonian, Mississippian, Pennsylvanian, and Permian populations diminish, these do not vanish compared to the underlying units (Wang et al. 2018b(Wang et al. , 2019b. This implies that both the NTS and CTS continue providing sediments for sedimentation in the Bogda Mountains. This is further evidenced by the detrital-zircon U-Pb age cumulative frequency curves and the probability plot, which exhibit zircon age groups like the syn-rift and post-rift sediments (Figs. 5 and 12). We attribute this to Late Triassic uplift of the Bogda Mountains, although the relatively smooth relief of its topography allowed scouring by rivers and consequently the supply of detritus from the NTS and CTS (Figs. 3 and 11).

Conclusions
Comparison of the detrital zircon U-Pb age distributions enabled constraining of the location of the Triassic magmatic activity in the SJB area. The comparison of the new detrital zircon U-Pb age dataset for the Middle-Upper Triassic Xiaoquangou Group of SJB shows that the Triassic magmatic activity occurred mainly in the Urumqi-Jimusaer area. The REEs abundance of the zircons reveal that the Triassic magmatism produced intermediate-acidic rocks. The geochronology data combined with provenance analysis indicates that provenance varies for different areas in SJB and adjacent areas, resulting in four sink areas. The Bogda Mountains started to supply sediments in the Late Triassic, marking the initial uplift of the Bogda Mountains. This study corroborates the utility of the comparison of detrital zircon U-Pb age distributions for identifying source locations. This suggests an effective approach for deciphering source characteristics when the potential source areas lack direct eld evidence of related detrital-zircon U-Pb ages.

Data availability
All data generated or analysed during this study are included in this published article (and its Supplementary Information les). Figure 1 Conceptual diagram of deep-time source-to-sink analysis using detrital minerals, especially detrital zircon U-Pb geochronology (modi ed after Romans et al. 2016

Supplementary Files
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