July 2008 LIP of the Month

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Early Cretaceous Comei Large Igneous Province: remnant identified in southeastern Tibet

Di-Cheng Zhu(1), Sun-Lin Chung(2), Xuan-Xue Mo(1), Zhi-Dan Zhao(1), Yao-Ling Niu(3), Biao Song(4)

(1) State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, 100083 Beijing, China (dchengzhu@163.com)

(2) Department of Geosciences, National Taiwan University, Taipei 106, Taiwan

(3) Department of Earth Sciences, Durham University, Durham DH1 3LE, UK

(4) Beijing SHRIMP II Center, Institute of Geology, Chinese Academy of Geological Sciences, 100037 Beijing, China

(The full story of the Comei LIP will be available in Zhu et al., to be submitted shortly).

The term "Large Igneous Province" (LIP) is used to represent a variety of mafic igneous provinces with areal extents > 0.1 Mkm2 (Coffin and Eldholm, 1994; Ernst et al., 2005; Bryan and Ernst, 2008) or > 0.05 Mkm2 (Sheth, 2007) as the result of "impact" of a mantle plume head at the base of the lithosphere mechanical boundary layer (Richards et al., 1989; Campbell and Griffiths, 1990; Coffin and Eldholm, 1994), or plume "incubation" that can cause substantial thermo-mechanical erosion of the overlying lithosphere (White and McKenzie, 1989; Kent et al., 1992; Sheth and Chandrasekharam, 1997). Early Cretaceous magmatism in southeastern Tibet (Fig. 1A) has been documented for years (BGMRXAR, 1993) and was recently ascribed to the consequence of plume–lithosphere interaction (Zhu et al., 2007, 2008). New geochronological and geochemical data have increased the known extent of the Early Cretaceous magmatism. Here we synthesize the results of field, petrographic, geochronological, geochemical studies of Early Cretaceous igneous rocks in SE Tibet (Fig. 1B), and we argue that these rocks are temporally related and constitute the erosional/deformational remnant of a large igneous province, which we call the Comei LIP.


Figure 1: The spatial extent of the Comei LIP in SE Tibet (Zhu et al., to be submitted)

Geological background

The Comei LIP is identified in the eastern segment of the Tethyan Himalaya, which is located between the Yarlung Zangbo suture zone to the north (YZS) and the Greater Himalaya to the south (GH) (Fig. 1A). The Tethyan Himalaya is dominated by marine sedimentary sequences over the period since the Paleozoic, and is generally considered to have been a typical passive continental margin on the northern edge of Greater India from the Late Triassic to the Early Cretaceous (Yu and Wang, 1990). The Tethyan Himalaya has also been interpreted to have developed in an extensional setting during continental breakup from the Middle–Late Paleozoic to the Early Cretaceous (Sciunnach and Garzanti, 1997; Garzanti et al., 1999; Zhu et al., 2007). The widespread mafic-dominated rocks exposed in southeastern Yamzho Yum Tso have been considered as an important component within the eastern Tethyan Himalayan sequence (BGMRXAR, 1993). However, there have been no good quality data on these rocks available until recently (Zhu et al., 2005, Jiang et al., 2006). which formed a basis for subsequent investigations (Zhu et al., 2007, 2008) that identified plume signatures in local areas (e.g., Rimowa village, and Cona areas) of southern Tibet. Like other pre-Cenozoic LIPs that have lost most of their volcanic components owing to tectonic deformation or erosion (Wingate et al., 2004; Ernst, 2007), the Comei LIP is dominated by dismembered mafic lava flows, sills and dikes, with subordinate ultramafic and silicic rocks of Late Jurassic–Early Cretaceous age, belonging to Sangxiu and Lakang Formations (Fig. 1B).

Field occurrence

Volcanic remnants
The remnant volcanic rocks are interbedded with the Late Jurassic–Early Cretaceous clastic sedimentary rocks in Sangxiu and Lakang Formations (Fig. 2A and 2B), which are distributed in the northern and southern parts of the Comei LIP (Fig. 1B). The volcanic rocks are bimodal, mostly basalts with some silicic varieties (Zhu et al., 2007). The basalts are generally massive; however, lavas with pillowed, columnar jointing structure (Fig. 2A) are also observed. The basalts vary in thickness from tens of meters to up to ~ 600 m (Zhu et al., 2008). The silicic rocks form a 130-m-thick succession of rhyodacite and rhyolite and show columnar jointing (Fig. 2B).


Figure 2: Field occurrence of the igneous rocks within the Comei LIP
(A). Field photograph showing the basalts of the Comei LIP are interbedded with slates of the Lakang Formation in Kada Gorge.
(B). Field photograph showing the relationship between the dacite and basalt of the Sangxiu Formation in Rimowa village.
(C). Field photograph showing diabasic dikes of the Comei LIP intruded into the Late Triassic sedimentary sequence near Pumo Yum Tso.

Mafic intrusions
Voluminous mafic intrusions intruded both the variably deformed Early Triassic–Lower Jurassic sedimentary strata that show rapid northward changes in thickness and lithofacies, and the partly deformed Jurassic–Early Cretaceous sedimentary strata that likely formed in a passive continental margin environment (Zhu et al., 2007) from the western Yamzho Yum Tso to southeastern Cona in SE Tibet (Fig. 1B). The mafic intrusions include diabasic sills, dikes, and gabbros (Fig. 2C). The sills and dikes extend generally in E–W direction, parallel to the strike of regional strata and also the Himalayan tectonic zone. Similar observations are presented for most gabbroic intrusions except for a few occurrences (e.g., Niangzhong village and Kada Gorge in the southeastern Comei LIP; Fig. 1B), where the gabbro intruded into an earlier wide diabasic sill. Ophitic and poikilitic textures are common in dike and gabbroic intrusions, respectively.

Ultramafic rocks
Ultramafic rocks include a ~80-m-thick layered olivine-pyroxenite (Fig. 3A and 3B) and ~ a 5-m-thick picrite porphyrite (Fig. 3C and 3D) that intruded the Jurassic sedimentary strata in the central Comei LIP and the Lakang Formation sedimentary rocks in the SE Comei LIP (Fig. 1B).


Figure 3: Field occurrence of the ultramafic rocks within the Comei LIP
(A-B). Layered olivine-pyroxenite observed in southern Chigu Tso of the central Comei LIP.
(C-D). Picrite porphyrite observed ~5km north of Gujue village, SE Comei LIP.

Zircon U-Pb geochronology

Zhu et al. (2005) first reported a SHRIMP zircon U-Pb age of 133 ± 3 Ma for a dacite sample in Rimowa village (Fig. 1B). Jiang et al. (2006) subsequently obtained two SHRIMP zircon U-Pb ages of 134.9 ± 1.8 Ma and 135.5 ± 2.1 Ma for two diabasic dike samples from the vicinity of Baidi village in the western Comei LIP (Fig. 1B). Zhu et al. (2008) recently reported two SHRIMP zircon U-Pb ages of 131.1 ± 6.1 Ma for a gabbroic intrusion in Kada Gorge and of 144.7 ± 2.4 Ma for a diabasic sill near Niangzhong village in the SE Comei LIP (Fig. 1B). An additional seven new SHRIMP zircon U-Pb ages ranging from 128 Ma to 137 Ma were obtained elsewhere in the Comei LIP (Fig. 1B). In total, 12 age determinations on rocks of the Comei LIP using the SHRIMP zircon U-Pb method have been obtained. Of the 12 age dates, 11 agree to within analytical uncertainty and have a weighted mean of 131.6 ± 2.5 Ma. These eleven age dates are from samples spanning a region as much as 270 km long and 150 km wide, i.e., the diabasic dikes in western Yamzho Yum Tso, the gabbros in Kada Gorge, and the gabbroic dikes to the south of Qonggyai County. One U-Pb age date on a diabasic sill near Niangzhong village (Fig. 1B) is significantly older (Zhu et al., 2008), suggesting that the mafic magmatism in this area may have commenced before the main magmatic event. These zircon U-Pb dates indicate that the bulk of the magmatic products in the Comei LIP were emplaced between ~ 145 Ma and 128 Ma with peak activity at ca. 132 Ma, covering an area of ~ 40, 000 km2 in southeastern Tibet (dashed ellipse in Fig. 1B). The diabases located further to the west (outside the ellipse) are similar and are also likely part of the Comei LIP, although this remains to be confirmed by dating.

Geochemistry

The mafic rocks in the Comei LIP can be subdivided into two major groups in terms of TiO2 and P2O5 contents, including the dominant high-Ti group (TiO2 > 2.0 %, P2O5 > 0.3 %) that consists of basaltic lavas, diabasic sills/dikes, and gabbroic intrusions, and the low-Ti group (TiO2 < 2.0 %, P2O5 < 0.2 %) that consists of basaltic lavas and gabbroic intrusions. The twelve SHRIMP zircon U-Pb age dates indicate that the high-Ti group persisted from ca. 145 Ma to 128 Ma, and that the tholeiitic magmatism occurred at 132 Ma, and continued to 128 Ma (Zhu et al., to be submitted).

From the 51 samples we exclude the altered and contaminated samples, which were identified on the basis of petrographic observations and geochemical diagnostic signatures, e.g., LOI content, and [Th/Nb]PM value, (PM is primitive mantle-normalized). From among the remaining samples, (87Sr/86Sr)t = 0.7042–0.7060, (143Nd/144Nd)t = 0.51250–0.51265, εNd(t) = 0.7–3.8 for seventeen high-Ti group samples which have Nb/Y ratios ranging from 0.53-1.07 (majority >0.68). Also (87Sr/86Sr) = 0.7040–0.7042, and (143Nd/144Nd)t 0.51258–0.51261, 2.1–2.7 for six layered picrite porphyrite samples with Nb/Y ratios ranging  from 0.5 to 0.7.. Zircons from the Dalong (DL06-1) and Dongjia (DJ3-4) gabbroic intrusions have εHf(t) values from +4.6 to +13.9, and from +1.5 to +19.9, respectively. It has been recognized that a long period of alkalic magmatism followed by enormous tholeiitic volcanism is typical of many flood-basalt provinces of the world (Sheth and Chandrasekharam, 1997). We therefore interpret the isotopic compositions of the uncontaminated high-Ti group (with alkalic to transitional nature and ultramafic samples) as reflecting the source characteristics of the Comei plume head, which is: (87Sr/86Sr)t = 0.7040–0.7060, (143Nd/144Nd)t = 0.51250–0.51265, εNd(t) = 0. 7–3.8.

Spatial extent of the Comei LIP

The Sangxiu Formation basalts exposed in Rimowa village are geochemically similar to Group 1 mafic rocks widely distributed in the Cona area (Zhu et al., 2007, 2008). This geochemical similarity is also shared by high-Ti group rocks elsewhere in SE Tibet (our unpublished data). While most outcrops of the Comei LIP (Fig. 1B) are undated, the field relationships and geochemical similarities of all these igneous rocks suggest that they are coeval and co-genetic. The Tethyan Himalaya is a tectonically active terrain in response to the India-Asia collision and continued convergence (Hodges, 2000). Therefore, significant tectonic shortening (Fig. 4A and 4B) accompanied by deep erosions has happened since the emplacement of the Comei LIP. This explains why the dominant rock assemblages here are deep level intrusives rather than basaltic flows. For all these reasons, we can infer with confidence that the Comei LIP must have occupied a substantially larger area (> 40, 000 km2).  during their emplacement probably exceeding the minimum level of 100,000 km2 for strict classification as a LIP (Coffin and Eldholm, 1994; Bryan and Ernst, 2008).


Figure 4: Field photographs showing significant shortening of the sedimentary sequence within the Comei LIP

A favored petrogenetic model

The older age date (~ 145 Ma) obtained from the Lakang Formation in NE Cona (Zhu et al., 2008) is close to the lowermost age of the Lakang Formation (Berriasian age, Xia et al., 2005), and therefore may represent the eruption age of undated basaltic lavas within these formations. This indicates potential precursor magmatism at ~145 Ma. The other 11 age dates covering an areal extent of 40, 000 km2 (Fig. 1B) suggest that the major magmatic event in the Comei LIP occurred at ~132 Ma. In this case, the Comei LIP can be interpreted as the results of two pulsed plume events including an older precursor at ~145 Ma and a short pulse at ~132 Ma; the two-pulse style is exhibited by many other LIPs around the world (Lin and van Keken, 2005; Bryan and Ernst, 2008).

However, another older age date (136.7 ± 1.1 Ma) obtained from ~ 5 km north of Comei County may imply that the magmatism in the Comei LIP initiated at ~145 Ma, and continued more continuously from 137 Ma to 128 Ma, instead of being pulsatory. Geochemical data indicate that the earlier magmatism is transitional-alkaline, which persisted from ca. 145 Ma to 128 Ma, whereas the tholeiitic magmatism occurred at 132 Ma, and continued to 128 Ma. The long-duration (~17 Myr magmatism, ~ 145–128 Ma) interpretation of the preliminary age data for the Comei LIP magmatism would be inconsistent with the plume head–tail model, which predicts a rather short period of magmatic emplacement, but would be consistent with the plume incubation model (White and McKenzie, 1989; Kent et al., 1992; Sheth and Chandrasekharam, 1997). In the latter model, the plume head incubates at the base of the lithosphere for more than ten million years, and produces alkaline magma in the initial stages of incubation. Our previous studies have indicated that contributions from the subcontinental lithospheric mantle were involved in generating the mafic rocks in the Sangxiu Formation and the Cona area, which was attributed to the effect of plume-lithosphere interaction (Zhu et al., 2007, 2008). We tentatively favor an interpretation of the relatively long duration of the magmatism in the Comei LIP (~17 Myrs) as reflecting the period of sub-lithospheric plume head incubation.

Acknowledgements

We greatly appreciate R.E. Ernst (Canada) for encouraging this contribution to the Large Igneous Province of the Month. Our ongoing study benefited from financial supports by the NSFC project (40503005, 40473020), and the National Key Project for Basic Research of China (Project 2006CB701402).

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