Discovery of the Dagele Eclogite in East Kunlun,Western China and Its Zircon SHRIMP U-Pb Ages:New Constrains on the Central Kunlun Suture Zone

Objective Eclogites are important indicators of ancient plate boundaries or paleosuture zones.Despite their great geological significance,very few investigations have been carried out in the Kunlun region.The Central East Kunlun fault zone was believed to be an Early Paleozoic suture zone,but there has been no reliable evidence for this,though studies on ophiolite,granite,and basic granulite indicate that the Early Paleozoic orogeny occurred in the East Kunlun.This work focused on the Dagele eclogites

The Age of the Original Silurian Badangshan Formation and its Ductile Deformation in the Northern Margin of North China Craton: New Evidence from Zircon SHRIMP U-Pb Ages

Objective Many previous researches have documented the tectonic evolution of the northern margin of North China Craton.However,whether the age of original Silurian Badangshan Formation in the study area belongs to the Archean,Silurian or Devonian remains controversial,and the ductile deformation time of this formation also lack

SHRIMP Zircon U-Pb Ages of the Wajilitag Igneous Complex: Constraints on the Origin of A-type Granitoids in the Tarim Large Igneous Province, NW China

Objective Large igneous provinces(LIPs)are sites of spatially contiguous,rapidly emplaced magmatic rocks,which represent the physical and chemical transfer of material from the mantle to the crust.Exposed within some continental LIPs are felsic and mafic plutonic and volcanic rocks.Although their volumes are minor compared to the flood basalts,the plutonic rocks of continental LIPs

SHRIMP Zircon U-Pb Age of the Sidingheishan Mafic-Ultramafic Intrusion in the Southern Margin of the Central Asian Orogenic Belt,NW China and its Petrogenesis implication

Objective The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the North Tianshan Mountains.This work used zircon U-Pb age data,bulk rock major and trace elements,Sr-Nd-Pb isotope data to assess mantle source characteristics and crustal assimilation of the parental magma of the Sidingheishan intrusion.We have also discussed the tectonic evolution of the southern margin of the Central Asian Orogenic belt in the Late Paleozoic.

New Zircon SHRIMP U-Pb Ages of the Langjiu Formation Volcanic Rocks in the Shiquanhe Area,Western Lhasa Terrane and their Implications

Objective The potassic and ultrapotassic rocks relating to the India-Eurasia collision and continual plate convergence are widely distributed in the Lhasa terrane.These rocks are very important to understand the deep processes of the India-Eurasia collision and the uplift and evolution of the Tibetan Plateau.Although high-potassic volcanic rocks are also exposed in the western Lhasa terrane,their

New detrital zircon U-Pb ages from BIF-related metasediments in the Ntem Complex (Congo craton) of southern Cameroon, West Africa

Banded Iron Formations (BIFs) were formed by contemporaneous events of active sediments supply and the venting of a hydrothermal fluid source at the Mid-Ocean-Ridge. BIFs within the Ntem Complex at the northern edge of the Congo Craton are intercalated with metasandstones and siltstones. SHRIMP U-Pb analysis on detrital zircons obtained from these metasediments gave variable ages from over 3000 Ma to 1000 Ma with the maximum age of deposition clustered around 1200 Ma and the peak of deposition at 1800 Ma. This age range suggested that the sub-basin was opened sometime in the Archean and remained active up till the Neoproterozoic. Zircons with Archean ages have a provenance linked to the charnockitic suite and the high-K granites within the Ntem Complex. The Paleoproterozoic ages are attributed to clastic inputs from the neigbouring Nyong Series west of the Ntem Complex. Also the peak of deposition in the Proterozoic could probably be explained by the globally recognized intense crust-forming processes in the Early Proterozoic time. The provenance of the younger Neoproterozoic ages is tied to various lithologies within the northern mobile belts of the Adamawa-Yade massifs and correlates with Neoproterozoic sedimentation ages in the Yaoundé, Lom and Poli series. The Neoproterozoic ages obtained are comparable to those obtained from metasediments of the Amazonian Craton and provide evidence of Pre-Gondwana assemblage of the Congo and the S?o Francisco Cratons.

Zircon SHRIMP U-Pb dating of the Neogene coral reefs,Xisha Islands,South China Sea:implications for tectonic evolution

The Xisha Block is a minor one in the South China Sea and an important tectonic unit in the northwestern part of the region.Zircon SHRIMP U-Pb ages for three volcanic intrusive core samples from Xike-1,an exploratory well penetrating the bioherms of the Xisha Islands.The core samples are from the Miocene reef carbonate bedrock and are recognized as dark-gray biotite-hornblende gabbro,gray fne-grained biotite diorite,and gray fine-grained granite,respectively.Zircon cathodoluminescence (CL)images and trace Th,U and Pb compositions of the zircons show that these rocks are of volcanic intrusive origin.Zircon SHRIMP U-Pb dating yielded six groups of ages,ranging from 2451-1 857 Ma to early Cretaceous, which indicate that the formation and evolution of the Xisha Block was affected by the evolution and closure of Neotethys Ocean,probably within its eastern extension into South China Sea.Both old,deepsourced material,including fragments from Rodina supercontinent,and recent mantle-derived magma products contributed to the emergence and formation of the Xisha block.The SHRIMP U-Pb results also proved that this process differed from that of the Kontum massif,the Hainan Block,and the South China Block,but is similar to that of the Nansha and Zhongsha blocks.The process was associated with the effects of Yanshanian magmatism induced by subduction mechanisms of the Paleo-Pacific Plate or the reworking of the multiple magmatisms since the Early to mid-Yanshanian,possibly jointly experienced by the Xisha-Zhongsha-Nansha Block.

The significance of U-Pb zircon ages in zoned plutons:the case of the Flamenco pluton, Coastal Range batholith, northern Chile

Compositionally zoned plutons, both layered and concentrically arranged, provide granitic exposures where the mechanisms and timing of the magmatic emplacement processes can be studied. The importance of in-situ geochemical differentiation and the magma replenishment rates are revealed by geochemistry and field relations, together with the increasingly accurate U-Pb geochronology, which has promoted the knowledge about the pluton incremental assembly theories.The Flamenco pluton, located in the Coastal Range of northern Chile, is part of the Upper Triassic to Early Cretaceous Andean intrusives formed in the western active margin of South America, and present a normal zoned structure with mafic magmatic facies(mostly gabbros and Qtz-diorites) close to the contacts with the host metasediments, and tonalites, granodiorites and granites in the inner areas. A combined study of the field relations, geochemistry and zircon geochronology of the magmatic facies was applied to determine the emplacement sequence of the Flamenco pluton, revealing three distinguishable domains separated by metasedimentary septa. The SW area is constituted by mostly homogeneous leucocratic granodiorites that yielded an age of 213 Ma as the best estimation for their emplacement age. Distinctive geochemical characteristics, such as the absence of an Eu anomaly, the depletion in HREE, or the highest Sr, Sr/Y and Ce/Yb values among the granodioritic facies of the pluton,involve lower T and/or higher P conditions at the magmatic source according to experimental studies.These conditions were established during an early stage of the Andean magmatic arc building that is firstly defined here as Upper Triassic. The NW and E domains of the pluton were sequentially emplaced between 194 Ma and 186 Ma and both the field relations and the detailed geochronological results suggest that the mafic facies intruded latter in the emplacement sequence. To the NW, Qtz-dioritic and gabbroic externally emplaced pulses gave a younger crystallization age of 186.3 ± 1.8 Ma, and promoted the granoblastic textures and metamorphic zircon overgrowths that characterize the granodiorites located in the contact with the intermediate and felsic inner magmas, which yielded a best estimation of their emplacement age of 192 士 1.5 Ma. On the other hand, in the eastern domain, magma-magma relations are observed between gabbros and previously intruded tonalites and granodiorites. Both the mafic and intermediate facies show two main subgroups of ages that yielded 194.7 土 1.5 Ma to188.3 ± 2.1 Ma and 193.1 ± 2.2 Ma to 185.5 ± 1.4 Ma respectively. These differences are related to the variations in the magmatic addition rates, which may extend the super-solidus conditions in the eastern domain of the magmatic reservoir as is confirmed by the wider age ranges yielded by these magmatic facies. Zircon overgrowths in the host rocks yield similar ages(around 220 Ma and 205 Ma) than the oldest results obtained in the intrusive facies, indicating that metamorphism correlates with the initial stages of plutonic emplacement.Geochronological results differ between 9 Myr and 41 Myr in the eight studied samples for noninherited ages and gave very close mean ages(within analytical uncertainty) for all the intrusive units. However, we examine other characteristics such as zircon morphology, internal structure,geochemistry and statistical data to assess if the scattering of the geochronological data may be related to the different processes involved in the construction of the Flamenco pluton. We concluded that this detailed study of U-Pb zircon ages, including individual and significative groups of analyses, is useful to determine accurately the emplacement sequence and the genetic relation between the intrusive units,together with the evidences depicted by the geochemistry and field relations.

中亚-蒙古造山带东段的锡林郭勒杂岩:早华力西期造山作用的产物而非古老陆块?——锆石SHRIMP U-Pb年代学证据

锡林郭勒杂岩是华北板块北缘古生代褶皱带内出露面积最大的变质岩系,以前多被当着前寒武纪的古老地块。本文通过对该杂岩中副片麻岩和正片麻岩的锆石SHRIMP U-Pb年代学研究发现,副片麻岩中的锆石多为岩浆锆石,其^(206)Pb/^(238)U加权平均年龄为406±7Ma,指示它们的原岩主要是由近同期(略早些)的岩浆岩风化后就近沉积的产物,该年龄应代表源区(岛弧型?)花岗岩的形成时间,同时也是副片麻岩原岩沉积的下限年龄。正片麻岩中岩浆锆石的^(206)Pb/^(238)U加权平均年龄为382±2Ma,代表花岗片麻岩原岩的侵位年龄。岩石中锆石的变质增生边的形成年龄为337±6Ma,代表锡林郭勒杂岩发生变质和变形的时间,该变质事件可能与贺根山缝合带内所发生的一次主要的碰撞造山作用有关。这些年龄资料充分说明,锡林郭勒杂岩并非古老地块,而是华力西早期岩浆作用、沉积作用和变质作用事件的产物.整个事件是在较短的时间范围内(～70Ma)完成的,推测该杂岩发育在碰撞造山带的弧前环境。中亚-蒙古造山带东南部(内蒙古的中、东部)碰撞前的构造格局可能不是典型的多岛洋体制,由于缺少古老的陆块,造山过程更多的表现为大洋的大陆化过程,即洋内俯冲形成岛弧,岛弧在被动大陆边缘拼贴聚合转化为新的大陆。

Geochronology and petrogeochemistry of Late Permian volcanic rocks in the B.Xiengnou area,Northwestern Laos:Petrogenesis and tectonic significance

The Nan Suture and Sukhothai Arc Terrane are products of the eastward subduction of the Paleotethyan Ocean during the Late Carboniferous to Triassic.However,their footprints in northwestern Laos are poorly constrained.New geochronological and geochemical data presented in this study demonstrate a Late Permian origin for the andesitic rocks in the B.Xiengnou area rather than Late Triassic.The breccia-bearing andesitic tuff in the B.On ultramafic complex yield a zircon U-Pb age of 260±1.4 Ma,geochemically displaying a MORB-like signature.The andesitic tuff in the B.Kiophoulan-B.Houayhak belt gave the U-Pb age of 254±1.3 Ma,with arc-like geochemical affinity.By combining geochronological and geochemical data from the Nan Suture and Sukhothai Arc Terrane,the authors suggest that the andesitic rocks in the B.On ultramafic complex formed in a back-arc basin background,which connected the Jinghong and Nan back-arc basin during the Permian;while the andesitic tuff in the B.Kiophoulan-B.Houayhak belt erupted in the Sukhothai continental arc setting.

Tectonic evolution of the Western Kunlun orogenic belt in northern Qinghai-Tibet Plateau:Evidence from zircon SHRIMP and LA-ICP-MS U-Pb geochronology

The Western Kunlun Range in northern Qinghai-Tibet Plateau is composed of the North Kunlun Terrane,the South Kunlun Terrane and the Karakorum-Tianshuihai Terrane. Here we report zircon SHRIMP and LA-ICP-MS U-Pb ages of some metamorphic and igneous rocks and field observations in order to pro-vide a better understanding of their Precambrian and Palaeozoic-early Mesozoic tectonic evolution. Based on these data we draw the following conclusions: (1) The paragneisses in the North Kunlun Terrane are likely of late Mesoproterozoic age rather than Palaeoproterozoic age as previously thought,representing tectonothermal episodes at 1.0―0.9 Ga and ～0.8 Ga. (2) The North Kunlun Terrane was an orogenic belt accreted to the southern margin of Tarim during late Mesoproterozoic to early Neopro-terozoic,the two episodes of metamorphisms correspond to the assemblage and breakup of Rodinia respectively. (3) The Bulunkuole Group in western South Kunlun Terrane,which was considered to be the Palaeoproterozoic basement of the South Kunlun Terrane by previous studies,is now subdivided into the late Neoproterzoic to early Palaeozoic paragneisses (khondalite) and the early Mesozoic metamorphic volcano-sedimentary series; the paragneisses were thrust onto the metamorphic vol-cano-sedimentary series from south to north,with two main teconothermal episodes (i.e.,Caledonian,460―400 Ma,and Hercynian-Indosinian,340―200 Ma),and have been documented by zircon U-Pb ages. (4) In the eastern part of the South Kunlun Terrane,a gneissic granodiorite pluton,which intruded the khondalite,was crystallized at ca. 505 Ma and metamorphosed at ca. 240 Ma. In combination with geochronology data of the paragneiss,we suggest that the South Kunlun Terrane was a Caledonian accretionary orogenic belt and overprinted by late Paleozoic to early Mesozoic arc magmatism.

SHRIMP zircon U-Pb age and significance of Early Paleozoic volcanic rocks in East Kunlun orogenic belt, Qinghai Province, China

Early Paleozoic volcanic rocks in Nuomuhong area occurred as basalt slice and meta-volcanic slice. SHRIMP zircon U-Pb dating of the basalt slice and the meta-volcanic slice show that the age of the basalt slice is 419±5 Ma, and that of the meta-volcanic slice is 401± 6 Ma. These ages directly testify that there existed Early Paleozoic ocean-continent transform in East Kunlun, the basalt slice was formed in an extensional mid-ocean ridge setting and the meta-volcanic rock slice was formed in an extrusion subduction and collision setting. The inherited zircon age of 1734 Ma in volcanic rocks reflects that the base of East Kunlun may be Middle Proterozoic.

SHRIMP U-Pb zircon age of the youngest exposed pluton in eastern China

Analysis using the Sensitive High Resolution Ion Microprobe (SHRIMP) shows that the Yongsheng pluton, located to the south of Huadian County, Jilin Province, in the North China Craton, has a zircon U-Pb isotopic emplacement age of (31.6±1.3) Ma. It is therefore the youngest exposed pluton so far recognized in eastern China. Although geochemical data indicate intensive crystal fractionation, the Sr-Nd isotopic features suggest that the magmatic source region of the lithospheric mantle was weakly depleted. This implies that the previous enriched lithospheric mantle had been replaced by juvenile asthenospheric mantle before or during the Palaeogene. This recognition has great significance for future work on the Mesozoic-Cenozoic geological evolution and deep geological processes in eastern China.

Maximum depositional age estimation revisited

In a recent review published in this journal,Coutts et al.(2019)compared nine different ways to estimate the maximum depositional age(MDA)of siliclastic rocks by means of detrital geochronology.Their results show that among these methods three are positively and six negatively biased.This paper investigates the cause of these biases and proposes a solution to it.A simple toy example shows that it is theoretically impossible for the reviewed methods to find the correct depositional age in even a best case scenario:the MDA estimates drift to ever smaller values with increasing sample size.The issue can be solved using a maximum likelihood model that was originally developed for fission track thermochronology by Galbraith and Laslett(1993).This approach parameterises the MDA estimation problem with a binary mixture of discrete and continuous distributions.The‘Maximum Likelihood Age’(MLA)algorithm converges to a unique MDA value,unlike the ad hoc methods reviewed by Coutts et al.(2019).It successfully recovers the depositional age for the toy example,and produces sensible results for realistic distributions.This is illustrated with an application to a published dataset of 13 sandstone samples that were analysed by both LA-ICPMS and CA-TIMS U–Pb geochronology.The ad hoc algorithms produce unrealistic MDA estimates that are systematically younger for the LA-ICPMS data than for the CA-TIMS data.The MLA algorithm does not suffer from this negative bias.The MLA method is a purely statistical approach to MDA estimation.Like the ad hoc methods,it does not readily accommodate geological complications such as post-depositional Pb-loss,or analytical issues causing erroneously young outliers.The best approach in such complex cases is to re-analyse the youngest grains using more accurate dating techniques.The results of the MLA method are best visualised on radial plots.Both the model and the plots have applications outside detrital geochronology,for example to determine volcanic eruption ages.

Zircon U-Pb geochronology of the Chinese continental crust: a preliminary analysis of the Elsevier science database

The amount of zircon U-Pb geochronological data for China has grown rapidly in recent years.Nearly 410,000 items of zircon U-Pb geochronological data,representing more than 7,000 relevant articles in the Elsevier Science Database,have been collected to a database in this research.Statistics on the ages and absolute errors of these collated data,yielded smallest standard errors for(206Pb/238U),(207Pb/235U),and(207Pb/206Pb)ages within respective time intervals of<1388.96 Ma,1388.96–3282.52 Ma,and>3282.52 Ma.The ages and their absolute errors were determined using three main geochronology methods,based on laser ablation inductively coupled mass spectrometer(LA-ICP-MS),sensitive high-resolution ion microprobe(SHRIMP),and secondary ion mass spectrometer(SIMS)measurements.We compared the influence of these different methods on errors for each age interval.In addition,using a Gaussian model of multi-peak fitting of zircon U-Pb age frequencies,we identified seven growth peaks in zircons from the Chinese continental crust,which are 48.60 Ma,131.49 Ma,249.91 Ma,444.27 Ma,835.95 Ma,1860.65 Ma,and 2505.54 Ma.It is clear that there are correspondences between these ages and various geological events,namely,the Wutai movement,Lvliang movement,Jinning movement,Caledonian movement,Indo-China movement,Yanshan movement,and Himalayaorogeny movement,respectively.The time and spatial distributions of these zircons correspond to distinct geological events on the Chinese continent,reflecting its crustal evolution.

SHRIMP single zircon U-Pb dating of the Kongling high-grade metamorphic terrain: Evidence for >3.2 Ga old continental crust in the Yangtze craton

Single zircons from two trondhjemitic gneisses and two clastic metasedimentary rocks without Eu anomaly of the Kongling high-grade metamorphic terrain are dated by the in situ SHRIMP U-Pb method. The results show that the trondhjemitic magma emplaced at 2947-2903 Ma. Concordant age of as old as 3.3 Ga is present in the detrital zircons from the clastic metasedimentary rocks. Together with the depleted mantle Nd model age (TDM =3.2-3.3 Ga) of the clastic metasedimentary rocks, this documents the presence of Paleoarchean continental crust in the Yangtze craton.

The Early Jurassic magmatism in northern Guangdong Province, southeastern China: Constraints from SHRIMP zircon U-Pb dating of Xialan complex

This paper reports SHRIMP zircon U-Pb ages of 196±2 Ma for granite, and 195±1 Ma for gabbro from the Xialan complex in the Meizhou area, northern Guangdong Province. These results shed new light on the calm stage of magmatic activity in southeastern China during 200-180 Ma, and revealed that the back-arc extension induced by the subduction of the western Pacific plate may have begun at 195 Ma at least. Field observation on the fresh outcrops allows us to recognize some features formed by magma mixing. A part of the gabbro has a fine-grained rim of 20-30 cm at the margin, and thins gradually to-ward the granite; numerous dark fine-grained to microcrystalline dioritic enclaves developed in the granite. These enclaves vary in shape and size, dark minerals concentrated at the margin of enclaves, and the contact between enclaves and host rock is either obvious or obscure, or gradational. In addi-tion, needle-shaped apatites are included in the enclaves. The REE patterns of gabbros, as well as the trace element patterns, are generally consistent with those of granitic rocks. The above characters further suggest that in the Early Jurassic the injection of basic magma had melted deep continental crust and produced acidic magma, and the Xialan complex was produced by the mixing of them.

Late Mesoproterozoic to Early Neoproterozoic Tectonic Evolution of the SW Yangtze Block, South China: Evidence from U-Pb Geochronology and Lu-Hf Isotopes of Detrital Zircons from Sedimentary Rocks

In situ zircon U-Pb geochronological and Lu-Hf isotope studies of detrital zircons from Late Mesoproterozoic to Early Neoproterozoic sedimentary units on the southwestern margin of the Yangtze Block have important implications for the tectonic evolution of the Yangtze Block.The Huili Group contains zircons whose ages are mainly Late Archean to Mesoproterozoic(2650-2450,2100-1800,and 1350-1150 Ma).The Dengxiangying Group has one major age population of 1900-1600 Ma,and two subordinate age populations of 1350-1100 and 2300-2000 Ma.Yanbian Group sedimentary rocks have a zircon age population mainly in the range of 970-850 Ma,contemporaneous with the ages of widespread arc-related magmatism in the western Yangtze Block.Combining these results with previous work,the Huili and Dengxiangying groups were most likely deposited during ca.1160 to 1000 Ma in an intra-continental rift basin setting,while the Yanbian Group accumulated during>920 to 782 Ma in a back-arc basin setting at the southwestern margin of the Yangtze Block.In addition,all these results further suggest a tectonic transition from a continental rift basin to a convergent environment at the southwestern margin of the Yangtze Block at 1000-970 Ma.

Origin of the Oligocene Tuolangla porphyry-skarn Cu-W-Mo deposit in Lhasa terrane,southern Tibet

Although some porphyry-skarn deposits occur in post-collisional extensional settings,the post-collisional deposits remain poorly understood.Here the authors describe the igneous geology,and mineralization history of Tuolangla,a newly-discovered porphyry-skarn Cu-W-Mo deposit in southern Tibet that belongs to the post-collisional class.The deposit is associated with Lower Cretaceous Bima Formation.It was intruded by granodiorite porphyry intrusions at about 23.1 Ma.Field investigation indicated that mineralization is spatially and temporally associated with granodiorite porphyry.Molybdenite yielded a Re-Os weighted mean age of 23.5±0.3 Ma and is considered to represent the age of skarn mineralization at the deposit.Theδ^34S values of sulfides,concentrated in a range between 0.6‰to 3.4‰,show that the sulfur has a homogeneous source with characteristics of magmatic sulfur.The Pb isotopic compositions of sulfides indicate that ore-forming metal materials were derived from the mantle and ancient crust.The granodiorite porphyry displays high SiO2(68.78%–69.75%)and K2O(3.40%–3.56%)contents,and relatively lower Cr(2.4×10^-6–4.09×10^-6),Ni(2.79×10^-6–3.58×10^-6)contents,and positiveεHf(t)values(7.7–12.9)indicating that the mineralization porphyry was derived from the partial melting of juvenile lower crust.The Tuolangla deposit is located in the central part of Zedang terrane.This terrane was once considered an ancient terrane.This terrane is in tectonic contact with Cretaceous ophiolitic rocks to its south and Mesozoic continental margin arc volcanics and intrusions of the Gangdese batholith of the Lhasa terrane to its north.Thus,the authors proposed that the Oligocene porphyry skarn Cu-W-Mo mineralization is probably associated with the Zedang terrane.This finding may clarify why the Oligocene(about 23 Ma)deposits are found only in the Zedang area and why mineralization types of the Oligocene mineralization are considerably different from those of the Miocene(17–14 Ma)mineralization.

Structure and deformation around the Gyirong basin, north Himalaya, and onset of the south Tibetan detachment system

Gyirong basin and its adjacent area are located at a special position in the Himalayan orogen, where the south Tibetan detachment system (STDS) and N-S trending rift converged. The north Himalayan orogen here can be divided into five petrologic-tectonic units successively from south to north: 1) the Greater Himalayan crystalline complex (GHC); 2) the STDS shear zone; 3) the Tethyan Himalayan sedimentary sequence (THS); 4) the late Cenozoic sedimentary basins, such as Gyirong and Oma basins; and 5) the Malashan gneiss dome. Structural studies show that this area experienced four stages of deformation: 1) the earlier south-directed thrusting, preserved both in the GHC and THS; 2) top-down-to-north slip along the STDS, normal faults related to this slip formed the early controlling structures of the Cenozoic basins,and the tilted pattern of the blocks between the basins indicated a north-directed slip; 3) east-west extension, the resultant N-S trending normal fault formed the eastern boundary of the basins; and 4) late gravitational collapse. Zircon SHRIMP U-Pb dating on the syn-deformational (leuco-) granite along the STDS indicates that the major activity of the STDS occurred at ca. 26 Ma, but its onset may have begun as early as ca. 36 Ma.