Ordo-Silurian assemblage in the Indochina interior: Geochronological, elemental, and Sr-Nd-Pb-Hf-O isotopic constraints of early Paleozoic granitoids in South Laos

Abstract

In order to verify the early Paleozoic accretionary assemblage in the Indochina interior and constrain the Prototethyan tectonic evolution in Southeast Asia, this study presents a set of new U-Pb geochronological, elemental, and Sr-Nd-Pb-Hf-O isotopic data for the fifty-two representative granitoids in South Laos. The granitoids from the Kontum terrane, Tam Ky-Phuoc Son tectonic zone, and southern Truong Son igneous zone in South Laos yield the crystallization ages of 464−485 Ma, 455−471 Ma, and 427−446 Ma, respectively, with a northerly younging trend within the Indochina interior. They are mainly monzogranite with A/CNK = 0.96−1.99 and K2O>Na2O, which are marked by enrichment in large-ion lithophile elements and depletion in high field strength elements with remarkable Nb-Ta, Sr-P, and Ti negative anomalies. Their initial 87Sr/86Sr ratios range from 0.70510 to 0.71559, εNd(t) from −9.5 to −3.0, (206Pb/204Pb)i from 18.65 to 19.72, (207Pb/204Pb)i from 15.66 to 15.80, and (208Pb/ 204Pb)i from 38.84 to 39.79. The corresponding zircon ɛHf(t) and δ18O values are in the range of −10.6 to +1.0 and 6.88‰ to 8.94‰, respectively. In addition, their Sr-Nd-Pb and Hf-O isotopic compositions are generally similar with those of time-equivalent granitoids in South Tibet and SW Yunnan, China, and synchronous mafic-intermediate igneous rocks in South Laos, but distinctive from those of the supracrustal sedimentary-derived South China Paleozoic granite and Lincang-Sukhothai S-type granite. The early Paleozoic granitoids in South Laos might have originated from a mixed source of the wedge-derived juvenile crust coupled with supracrustal materials. All these data synthetically suggest the southward subduction of the Tam Ky-Phuoc Son Ocean and the northerly on-growing Ordo-Silurian accretionary orogenesis within the previously defined “single-ancient” Indochina block. The assemblage of the Indochina block might initiate at ca. 430 Ma in the Silurian and terminate in the Early-Middle Devonian.

Prototethyan Accretionary Orogenesis Along the East Gondwana Periphery: New Insights From the Early Paleozoic Igneous and Sedimentary Rocks in the Sibumasu

“…Dong et al, 2011;Myrow et al, 2009;Usuki et al, 2009Usuki et al, , 2013Veevers et al, 2005;Wang et al, 2010Wang et al, , 2016. Their ε Hf (t) values for the ∼550 Ma and ∼950 Ma detrital grains range from −11.8 to +9.2 and show similar patterns with the Himalaya-Qiangtang and eastern Indochina of the India-Antarctica segment of East Gondwana (Figures 6d-6f and 11; e.g., Burrett et al, 2014;Veevers et al, 2005;Wang et al, 2020aWang et al, , 2020bWang et al, , 2020cZhu et al, 2011). In addition, our detrital zircons are commonly sub-rounded or rounded in shape, reflective of relatively long-distant transportation.…”

“…A northerly propagating accretionary orogen, from ∼485 Ma in the Kontum to ∼430 Ma in the northern Truong Son zone, was proposed in response to the closure of the early Paleozoic Tam Ky-Phuoc Son Ocean (likely equivalent to the Yunxian-Menghai Ocean) and subsequent assemblage (e.g. Wang et al, 2020aWang et al, , 2020bWang et al, , 2020c. As a result, the early Paleozoic Yunxian-Menghai Ocean in the SW Yunnan and SE Asia portions of the Sibumasu is compatible with the Tam Ky-Phuoc Son Ocean in the eastern Indochina, corresponding to two segments of the Prototethyan Ocean.…”

“…(d) Apparent age(Ma) versus ε Hf (t), and (e-f) ε Hf (t) frequency for the 500-600 Ma and 900-1,100 Ma detrital grains from the Sibumasu, Himalaya-Qiangtang, and eastern Indochina, respectively. Data are from this study,Veevers et al (2005),Zhu et al (2011), Dong, Li, et al (2011), Zhang et al (2018;Wang et al (2018Wang et al ( , 2020aWang et al ( , 2020c.…”

“…Rb/Sr versus Rb/Ba (a), Al 2 O 3 /TiO 2 versus CaO/Na 2 O (b) (fromSylvester, 1998), Hf-Rb/30-Ta*3 (c) and Y versus Nb (d) for the early Paleozoic granites in the SW Yunnan and SE Asia portions of the Sibumasu. Also shown the synchronous granitoids from eastern Indochina (e.g.,Wang et al, 2020aWang et al, , 2020bWang et al, , 2020c.…”

“…Synthetic histograms of detrital zircon U-Pb apparent ages for the early Paleozoic sedimentary rocks from the (a) Sibumasu, eastern Indochina, west Cathaysia and Yangtze, (b) Tethyan-Himalaya, South Qiangtang and eastern Indochina, and (c) Phanerozoic sedimentary rocks from the Sumatra, West Australia and Lhasa blocks, respectively. Data are from this study,Cawood and Nemchin (2000),Veevers et al (2005),Wang et al (2010Wang et al ( , 2020aWang et al ( , 2020bWang et al ( , 2020c,Yao et al (2011), Dong, Li, et al (2011), Zhu et al (2011), Burrett et al (2014), Cai et al (2017), and Zhang et al (2018.…”