The Pamir Plateau, a result of the India‐Asia collision, contains extensive exposures of Cenozoic middle to lower crust in domes exhumed by north‐south crustal extension. Titanite grains from 60 ...igneous and metamorphic rocks were investigated with U‐Pb + trace element petrochronology (including Zr thermometry) to constrain the timing and temperatures of crustal thickening and exhumation. Titanite from the Pamir domes records thickening from ~44 to 25 Ma. Retrograde titanite from the Yazgulem, Sarez, and Muskol‐Shatput domes records a transition from thickening to exhumation at ~20–16 Ma, whereas titanite from the Shakhadara dome records prolonged exhumation from ~20 to 8 Ma. The synchronous onset of exhumation may have been initiated by breakoff of the Indian slab and possible convective removal of the Asian lower crust and/or mantle lithosphere. The prolonged exhumation of the Shakhdara and Muztaghata‐Kongur Shan domes may have been driven by continued rollback of the Asian lithosphere concurrent with shortening and northwestward translation of the Pamir Plateau.
Key Points
Titanite ages record different durations of gneiss dome exhumation from 20 to 8 Ma
Independent exhumation was driven by two tectonic events rather than GPE alone
Titanite retained Pb and Zr > 650 C for ~25 Myr
Surface wave tomography shows that the central Tibetan Plateau (the Qiangtang block) is characterized by S wave speeds as slow as 3.3 km/s at depths from 20–25 km to 45–50 km and S wave radial ...anisotropy of at least 4% (VSH > VSV) that is stronger in the west than the east. The depth of the Curie temperature for magnetite inferred from satellite magnetic measurements, the depth of the α‐β quartz transition inferred from VP/VS ratios, and the equilibration pressures and temperatures of xenoliths erupted from the middle to deep crust indicate that the Qiangtang crust is hot, reaching 1000°C at the Moho. This inferred thermal gradient crosses the dehydration melting solidi for crustal rocks at 20–30 km depth, implying the presence or former presence of melt in the Tibetan middle to deep crust. These temperatures do not require the wholesale breakdown of mica at these depths, because F and Ti can stabilize mica to at least 1300°C. Petrology suggests, then, that the Qiangtang middle to deep crust consists of a mica‐bearing residue from which melt has been extracted or is being extracted. Wave speeds calculated for mica‐bearing rocks with a subhorizontal to gently dipping foliation and 2% silicate melt are a good match to the wave speeds and anisotropy observed by seismology.
Key Points
Slow S wave speeds in Tibet require the presence of a few percent partial melt
S wave anisotropy in Tibet requires subhorizontally oriented mica
The Mogok metamorphic belt (MMB) extends for over 1,000 km along central Burma from the Andaman Sea to the East Himalayan syntaxis and represents exhumed lower and middle crustal metamorphic rocks of ...the Sibumasu plate. In the Mogok valley region, the MMB consists of regional high‐grade marbles containing calcite + phlogopite + spinel + apatite ± diopside ± olivine and hosts world class ruby and sapphire gemstones. The coarse‐grained marbles have been intruded by orthopyroxene‐ and clinopyroxene‐bearing charnockite‐syenite sheet‐like intrusions that have skarns around the margins. Syenites range from hornblende‐ to quartz‐bearing and frequently show layering that could be a primary igneous texture or a later metamorphic overprint. Calc‐silicate skarns contain both rubies and blue sapphires with large biotites. Rubies occur in marbles with scapolite, phlogopite, graphite, occasional diopside, and blue apatite. Both marbles and syenites have been intruded by the Miocene Kabaing garnet‐muscovite‐biotite peraluminous leucogranite. New mapping and structural observations combined with U‐Th‐Pb zircon, monazite, and titanite geochronology from syenites, charnockites, leucogranites, meta‐rhyolite‐tuffs, and skarns have revealed a complex multiphase igneous and metamorphic history for the MMB. U‐Pb zircon ages of the charnockite‐syenites fall into three categories, Jurassic (170–168 Ma), latest Cretaceous to early Paleocene (~68‐63 Ma), and late Eocene–Oligocene (44–21 Ma). New ages from five samples suggest that metamorphism in the presence of garnet and melt occurred between ~45 and 24 Ma. U‐Pb titanite ages from the ruby marbles and meta‐skarns at Le Oo mine in the Mogok valley are 21 Ma, similar to titanite ages from an adjacent syenite (22 Ma). U‐Th‐Pb dating shows that all the metamorphic ages are Late Cretaceous–early Miocene and related to the India‐Sibumasu collision.
Key Points
Rubies and sapphires in granulite‐facies marbles from the Mogok metamorphic belt, Myanmar, are spatially associated with charnockite‐syenite sill‐like intrusions and surrounding skarns
U‐Th‐Pb LA‐ICPMS dating of zircon, monazite, and titanite shows that there were two groups of charnockite‐syenite dates, one Jurassic in age (170–168 Ma) and one latest Cretaceous to early Miocene (~68–21 Ma)
Regional granulite‐facies metamorphism along the Mogok metamorphic belt is Late Cretaceous to Oligocene or early Miocene in age (~68‐21 Ma), peaking with garnet‐present melting between 45 and 21 Ma
The thermal structure of the Tibetan plateau—the largest orogenic system on Earth—remains largely unknown. Numerous avenues provide fragmentary pressure/temperature information, both at the present ...(predominantly informed though geophysical observation) and on the evolution of the thermal structure over the recent past (combining petrological, geochemical, and geophysical observables). However, these individual constraints have proven hard to reconcile with each other. Here, we show that models for the simple underthrusting of India beneath southern Tibet are capable of matching all available constraints on its thermal structure, both at the present day and since the Miocene. Many parameters in such models remain poorly constrained, and we explore the various trade‐offs among the competing influences these parameters may have. However, three consistent features to such models emerge: (i) that present‐day geophysical observations require the presence of relatively cold underthrust Indian lithosphere beneath southern Tibet; (ii) that geochemical constraints require the removal of Indian mantle from beneath southern Tibet at some point during the early Miocene, although the mechanism of this removal, and whether it includes the removal of any crustal material, is not constrained by our models; and (iii) that the combination of the southern extent of Miocene mantle‐derived magmatism and the present‐day geophysical structure and earthquake distribution of southern Tibet require that the time‐averaged rate of underthrusting of India relative to central Tibet since the middle Miocene has been faster than it is at present.
Plain Language Summary
Numerous proxies for the temperature structure of the lithosphere beneath southern Tibet exist. Geophysical observations provide information about the present‐day structure, geometry, and thermal structure of the area; geodetic and geological observations constrain the regional kinematics over the recent past; geochemical and petrological observations from magmatic rocks within the Himalaya‐Karakorum‐Tibetan plateau provide sparse information about the temperatures beneath southern Tibet over the history of the collision zone. In this paper, we summarize the range of these observations that are presently available and present a series of thermal models aimed at exploring what parameters for the thermal evolution of southern Tibet are capable of explaining all of these observations within a coherent framework. While present‐day observations require the presence of a cold India‐derived lithosphere underthrust beneath southern Tibet, geochemical and petrological data require that this was not present during the early Miocene and was removed prior to this. Further, to fit all observations, our models require that the average rate of underthrusting since the middle Miocene must have been greater than that observed at the present.
Key Points
We summarize available geophysical and petrological proxies for the thermal structure of southern Tibet
Simple two‐dimensional thermal models are capable of matching the majority of available constraints
Average convergence across the Himalayas must have been faster than the present‐day rate
A method for depth profiling using single-shot laser-ablation split stream (SS-LASS) ICP-MS is developed to simultaneously measure U-Pb age and trace-element concentrations in titanite. Simple ...semi-infinite, 1-D half-space diffusion models were applied to near-rim, trace-element zoned domains in titanite to distinguish between cooling and (re)crystallization ages and investigate the potential for preservation of thermally mediated diffusive loss profiles. These data illustrate the need to measure multiple trace elements with varying diffusivities to interpret a mineral's thermal history resulting from the non-unique nature of 1-D diffusion models where both temperature and time are unknown. A case study of titanites from two Pamir Plateau gneiss domes indicates they underwent ≥25 Myr of amphibolite and granulite facies metamorphism yet did not experience significant volume diffusion modification following (re)crystallization. The interpretation of prolonged (re)crystallization rather than diffusion allows for high-resolution, near-rim temperature-time histories to be extracted using U-Pb dates and Zr4+ apparent temperatures by SS-LASS.
•SS-LASS is a high-resolution technique for measuring chemical and isotopic zoning concurrently in the rims of titanite.•Simultaneously collected U-Pb isotopic dates and Zr thermometry enables the calculation of thermal histories from the rims of zoned titanite.•Simple 1D diffusion modeling of multiple element pairs with varying diffusivity allows for insight into mineral closure versus (re)crystallization in titanite•Depth profiling and simple diffusion modeling of the near-rim zoning in titanites from the Pamir Plateau record (re)crystallization rather than diffusional modification.
The western Tianshan high‐pressure/low‐temperature orogenic belt in NW China contains eclogite‐facies metavolcanic rocks and omphacite‐bearing blueschists. Previous Sm‐Nd (omphacite, garnet, ...glaucophane, whole rock) and40Ar/39Ar (crossite) dating of eclogite‐facies rocks has suggested an age of ca. 345 Ma as the best approximation for the timing of peak metamorphic conditions. The samples described here are blueschist‐facies rocks that formed during or after the transition from the eclogite‐facies to the epidote‐blueschist‐facies and subsequently experienced an incipient greenschist‐facies overprint. By use of white mica geochronology (K‐Ar,40Ar/39Ar, Rb‐Sr), an attempt is made to date postpeak metamorphic stages of the complexPTpath. Rb‐Sr and40Ar/39Ar ages range between 313 and 302 Ma and 323 and 312 Ma, respectively, but mostly cluster at ca. 310–311 Ma, indicating that the studied samples recrystallized at this time. However, the40Ar/39Ar age spectra show complex release patterns that are interpreted to be influenced by excess argon to varying degrees. This conclusion is further corroborated by K‐Ar dates ranging between 385 and 309 Ma. Younger dates of ca. 302 Ma (Rb‐Sr) and ca. 296 Ma (K‐Ar) indicate subsequent disturbances of these isotope systems in some rocks. The new ages are significantly younger than the time of eclogite‐facies metamorphism (350–345 Ma), indicating resetting of the40Ar/39Ar, K‐Ar, and Rb‐Sr systems during exhumation of the blueschist‐facies rocks. Furthermore, this dataset suggests that high‐pressure conditions were attained during the Carboniferous and not at Permian or Triassic time, as recently suggested by SHRIMP U‐Pb zircon dating.
The Woodlark Rift in Papua New Guinea hosts the world's youngest (2–8
Ma) eclogite-facies rocks and extensional deformation has played a key role in exhuming these (U)HP rocks at rates of >
20
mm/yr. ...During the Eocene Papuan arc-continent collision Australian Plate-derived continental rocks were subducted to (U)HP depths. There they remained for up to 30
m.y. until the Pliocene when asthenospheric circulation ahead of the west-propagating Woodlark spreading ridge introduced heat and fluids. This caused rocks to break away from the paleosubduction channel, recrystallize in the eclogite facies, and rise as Rayleigh–Taylor instabilities. The diapirs ascended adiabatically undergoing partial melting, which lowered their viscosity and increased buoyancy. (U)HP crust ponded near the Moho at ~
2–4
Ma, thickening the crust to ~
40
km (11
kb). Domal uplifts emerged above sea level, and these are still underlain by an unusually thick crust (>
26
km) for a rift that has stretched by factor of ~
3 since 6
Ma. After ponding, they acquired a flat-lying foliation during amphibolite-facies retrogression. Vertical shortening accompanied the gravitationally driven outflow of ponded lower crust. The weak material was extended parallel to the rift margin, thinning ductilely by <
1/3. The flow was dominated by pure shear (
W
k
~
0.2), and was mechanically decoupled from – and orthogonal to – plate motion in the rift. Top-E shear fabrics suggest that this flow was westward, perhaps driven by isostatic stresses towards a strongly thinned rift corridor ahead of the Woodlark spreading ridge. At <
2
Ma, the gneisses were upwardly juxtaposed against an ophiolitic upper plate to form nearly symmetric gneiss domes that cooled at >
100
°C per m.y. and were mechanically incorporated into the rift's upper crust. Final exposure was by normal faulting and minor erosion. Such exhumation may also apply to other (U)HP terranes where less evidence for Moho ponding is preserved.
► Rocks stay cool in dead subduction zone for >
20
m.y., then UHP-crystallized as result of heating. ► Partially molten diapirs rise at >
2
cm/yr to form World's youngest (U)HP terrane. ► Diapirs ponded at Moho, then flowed outward under gravity, parallel to rift margin. ► Ductile flow was pure-shear dominant; thinning caused 10
km of exhumation. ► Rocks emplaced into upper crust as gneiss domes; exposed by normal faulting.
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