HIMALAYAN MAFIC MAGMATISM: PETROGENETIC CONSTRAINTS ON THEIR SOURCES AND ATTENDENT TECTONICS
|گروه||سازمان زمین شناسی و اکتشافات معدنی کشور|
|مکان برگزاری||یازدهمین گردهمائی علوم زمین|
|تاريخ برگزاری||۲۴ بهمن ۱۳۷۱|
Himalaya is subdivided into four principal tectonic zones from south towards north these are the Sub Himalaya, the Lesser Himalaya, the Greater or Higher Himalaya and the Tethys Himalaya. It has a record of pre- orogenic mafic magmatism widely distributed in space and time. Volcanism from Proterozoic to Ordovician appear to have been restricted to western and central parts of the Lesser Himalaya, whereas late Permian to Cretaceous it is recorded in the whole region although the focus of magmatism shifted from Lesser to Tethyan Himalaya. The initial phases of magmatism are of early Proterozoic (۲.۵۱ Ga old) period which are represented by the Mandi- Darla and Rampur volcanics of Himachal Pradesh, Garhwal volcanics of Garhwal region and Bhovali- Bhimtal volcanics of Kumaun regions of the Lesser Himalaya. After these Proterozoic volcanism there was a period of quiescence until the Ordovician period when Bafliaz volcanics erupted further northwest in the Punch-Rajauri regions of Jammu and Kashmir. This was followed by a major volcanic eruption during the Permo-Triassic period in the form of Panjal and Phe volcanics in Kashmir and Zanskar region of the Tethys Himalaya. These were followed by the Triassic- Jurassic lamayuru volcanics in the northwest and Abor volcanics in the eastern Himalaya. The Cretaceous activity being represented by the ophiolitic rocks of the Indus- Yarlung-Zangbo suture zone.
In summary the Himalayan volcanics are mostly volcano- sedimentary supracrustal sequences. They have undergone greenschist to amphibolite grade of metamorphism. Their primary mineralogy of plag + cp x + ol is partially replaced by secondary minerals such as amphiboles, chlorite and serpentine etc. Other common minor, accessory and secondary minerals are epidote, Calcite, quartz, biotite, Fe- Ti oxides, sphene etc.
Classifications based on relatively less mobile major and trace elements reveal that the volcanics are predominantly tholeiitic during the Proterozoic, through minor alkaline although predominantly tholeiitic during Ordovician. Bafliaz volcanism to predominantly alkaline with minor tholeiites in the younger phases of Panjal, Phe, Lamayuru and Abor volcanics.
To illustrate the general characteristics of these volcanics, normalised rare earth and incompatible trace elements patterns (spidergrams) are considered. Emphasis is put only on those elements which are known to be relatively less mobile during alteration and metamorphism.
Chondrite normalised rare earth element (REE) patterns for the Proterozoic volcanics show enriched REE characteristics with high and variable LREE/ HREE ratios. The (Ce/ Yb) N varies between ۳.۳ and ۸.۹ for Garhwal volcanics; ۳.۱ and ۵.۸ for Mandi- Darla and ۳.۶ and ۴.۷ for Bhowali- Bhimtal volcanics. The observed large variations in these ratios even within individual volcanic suites, suggest that all samples cannot be related to one parental melt by simple fractional crystallisation processes, it implies rather more complicated genesis. The spidergram patterns show general enrichment of large ion lithophyle elements (LlLE) with distinct negative anomalies for high field strenght elements (HFSE) such as Nb, P and Ti. Variable negative anomalies for Eu and Sr could be related to plagioclase fractionation and/or their mobility. Most of the continental flood basalts (CFBs) and Proterozoic dyke swarms also have these chemical characteristics i.e. enrichment of LILE- LREE and depletion of HFSE apart from having rather higher SiO۲ and Fe/ Mg ratios, as also seen in all these Himalayan Proterozoic volcanics.
Normalised REE patterns for the Bafliaz volcanics does show the LREE enriched characteristics, the alkaline rocks have much more fractionated REE patterns with (Ce/ Yb) N between ۹.۲۸ and ۱۶.۷ and positive Eu anomalies. The tholeiitic REE patterns either have slightly negative or no Eu anomalies and their (Ce/ Yb) N vary between ۴.۲۴ and ۶.۶۳. In terms of spidergram patterns the Bafliaz tholeiites have identical signature as noted earlier for the Proterozoic volcanics except for P. The alkaline rocks show much higher enrichment for LILE and LREE but they also have strong positive anomalies for Nb, P and Tl They have much higher Ti/ Y ratios (>> ۱) compared to the contemporary tholeiites with Ti/ Y ratios < < ۱.
Tholeiites and alkaline rocks of the next younger panjal and Phe volcanics have similarly enriched REE patterns with (Ce/ Yb) N varying between ۲.۷۵ and ۸.۶۹ in the case of Phe volcanics but the difference in the fractination of the REE patterns observed between alkaline and tholeiites of the Bafliaz volcanics is not apparent in these younger volcanics, it changes rather gradually, although the total REE abundance is much higher in case of the alkaline rocks. This would imply that it is not possible to generate both tholeiite and alkaline rocks from the same source in case of the Bafliaz but it is possible in case of the Panjal and Phe volcanics. In the spidergram patterns most of the trace element characteristics remain same as those discussed for the earlier tholeiites, however, Ti anomaly is absent in case of Panjal and Phe tholeiites as also these rocks have higher Ti/ Y ratios (> ۱) compared to the older ones with < ۱. The trace element characteristics of the alkaline rocks remain same as those noted for the Bafliaz alkaline volcanics.
The younger Lamayuru and Abor volcanics have similarly enriched characteristics for the alkaline and tholeiitic rocks but they have much higher Ti/ Y ratios. The REE patterns for the Abor volcanics show large variations in their (Ce/ Yb) N which varies between ۲.۱۸ and ۴.۸۴ but the patterns do not have any Eu anomaly.
The spidergram patterns also show no significant anomalis for Nb, P and Ti but for Sr which is very strongly negative. In the absence of any Eu anomaly, plagioclase fractionation is discounted implying that the Sr anomalies could be related to their mobile nature.
The observed trace element enriched characteristics of different volcanic phases in Himalaya could be explained in terms of crustal contamination of mantle derived melts but a critical review of the data overwhelmingly indicate that the compositional characteristics have been inherited from their metasomatised mantle sources.
Barring a few small differences a distinct overall similarity is evident in the chemical characteristics of the alkaline rocks of Himalaya and those of initial rift tholeiites (IRT) and ocean island basalt (OIB) with positive anomalies for HFSE (Nb, P and Ti) apart from enriched LREE and LILE. The tholeiites displaying similarities with those of the continental flood basalt (CFB) and Proterozoic dyke swarms observed earlier.
The observed changes in chemistry of the volcanics from predominantly tholeiitic with CFB and dyke swarm signatures for the older volcanics to predominantly alkaline with IRT and OIB signatures for the younger volcanics imply an interrupted extensional environment. It is inferred that a mature continental lithosphere got mobilised and partially melted due to thermal episodes/ deeper mantle plumes under extensional regime to generate the older tholeiites. Later on the lithosphere got attenuated giving way to asthenospheric material/ mantle plume to erupt, although without much interaction with continental lithosphere, generating predominantly alkaline rocks and secondary tholeiites.
As it is generally accepted that Alpine- Himalayan belt evolved due to common processes and the fact that Iran has a much better volcanic record especially for Paleozoic and Mesozoic periods, it is suggested that a detailed study of the Phanerozoic volcanics of Iran, in combination with the data available from Himalaya and Alps, may help better understand changes that the mantle might have undergone from Proterozoic to Mesozoic periods apart from putting much better constraints on global tectonics.