H) Geochemistry:

        As mentioned in the geology section, a considerable amount of volcanism occurred after the initiation of the orogen. Older volcanism (60 - 30 Ma) is mainly composed of high potassium lavas and calc-alkaline dacites, trachytes and ryholites. Petrologic studies suggest that they originate from partial melting of crust and lithospheric mantle. On the other hand younger highly potassic volcanism (<20 Ma) distributed along Altyn-Tagh and Kunlun faults, show small fractional melting of mantle lithosphere (Arnaud et al. 1992). Although the genesis of volcanism was commonly related to a sudden tectonic process such as convective removal of the mantle lithosphere, it is more consistent with melt sources related to subduction. To understand how valid the proposed models are it is essential to quantify thermal gradients. For this purpose, xenoliths which are trapped and later taken out by volcanic activity can be useful.
        Recently discovered and studied xenoliths in Tibetan plateau (figure 21) revealed that the lower crust includes anhydrous metasedimentary granite facies equilibrated at high temperatures (800 to 1100 C) at depths of 30 to 50 km (Hacker et al. 2000)(figure 22). Observed metasedimentary composition contradicts with convective thinning of lithospheric mantle (delamination). Moreover, the calculated seismic velocities and Poisson's Ratios for melted anhydrous mineral content match with seismic observations and suggest the absence of widespread partial melt (figure 22). But taking into account that xenoliths do not show geological relationships, nobody knows how representative they are.
 

Figure 21. Volcanic rocks and xenolith localities on the Tibetan plateau (xenoliths: asterisk) [Hacker et al. 2000].
 
 
 

Figure 22. Tibetan xenolith thermobarometry and calculated P-wave velocities [Hacker et al. 2000].
 
 
 

 

---

 

A) Tectonics & Geology

B) Seismicity

C) GPS

D) Seismic profiles & Tomography

E) Gravity

F) Anisotropy

G) Paleomagnetism

H) Geochemistry

I) MT studies

 

---

Home

Introduction

Paleotectonic Evolution

Recent Deformation

Models

Discussion

References

---