Tectonic-Igneous Associations

Transcription

1 Tectonic-Igneous Associations Associations on a larger scale than the petrogenetic provinces An attempt to address global patterns of igneous activity by grouping provinces based upon similarities in occurrence and genesis

2 Tectonic-Igneous Associations Mid-Ocean Ridge Volcanism Ocean Intra-plate (Island) volcanism Continental Plateau Basalts Subduction-related volcanism and plutonism Island Arcs Continental Arcs Granites (not a true T-I Association) Mostly alkaline igneous processes of stable craton interiors Anorthosite Massifs

3 Chapter 13: Mid-Ocean Rifts The Mid-Ocean Ridge System Figure After Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36,

4 Ridge Segments and Spreading Rates Slow-spreading ridges: < 3 cm/a Fast-spreading ridges: > 4 cm/a are considered Temporal variations are also known

5 Ridge Segments and Spreading Rates Hierarchy of ridge segmentation Deval OSC OSC = overlapping spreading center Deval = deviation from axial linearity Figure S1-S4 refer to ridge segments of first- to fourth-order and D1-D4 refer to discontinuities between corresponding segments. After Macdonald (1998).

6 Oceanic Crust and Upper Mantle Structure 4 layers distinguished via seismic velocities Deep Sea Drilling Program Dredging of fracture zone scarps Ophiolites

7

8 Oceanic Crust and Upper Mantle Structure Typical Ophiolite Figure Lithology and thickness of a typical ophiolite sequence, based on the Samial Ophiolite in Oman. After Boudier and Nicolas (1985) Earth Planet. Sci. Lett., 76,

9 Oceanic Crust and Upper Mantle Structure Layer 1 A thin layer of pelagic sediment Figure Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.

10 Oceanic Crust and Upper Mantle Structure Layer 2 is basaltic Subdivided into two sub-layers Layer 2A & B = pillow basalts Layer 2C = vertical sheeted dikes Figure Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.

11 Layer 3 more complex and controversial Believed to be mostly gabbros, crystallized from a shallow axial magma chamber (feeds the dikes and basalts) Layer 3A = upper isotropic and lower, somewhat foliated ( transitional ) gabbros Layer 3B is more layered, & may exhibit cumulate textures

12 Oceanic Crust and Upper Mantle Structure Discontinuous diorite and tonalite ( plagiogranite ) bodies = late differentiated liquids Figure Lithology and thickness of a typical ophiolite sequence, based on the Samial Ophiolite in Oman. After Boudier and Nicolas (1985) Earth Planet. Sci. Lett., 76,

13 Layer 4 = ultramafic rocks Ophiolites: base of 3B grades into layered cumulate wehrlite & gabbro Wehrlite intruded into layered gabbros Below cumulate dunite with harzburgite xenoliths Below this is a tectonite harzburgite and dunite (unmelted residuum of the original mantle)

14 Elevation of ridge reduces with time as plate cools

15 Petrography and Major Element Chemistry A typical MORB is an olivine tholeiite with low K 2 O (< 0.2%) and low TiO 2 (< 2.0%) Only glass is certain to represent liquid compositions

16 The common crystallization sequence is: olivine (± Mg- Cr spinel), olivine + plagioclase (± Mg-Cr spinel), olivine + plagioclase + clinopyroxene Figure 7.2. After Bowen (1915), A. J. Sci., and Morse (1994), Basalts and Phase Diagrams. Krieger Publishers.

17 Fe-Ti oxides are restricted to the groundmass, and thus form late in the MORB sequence Figure 8.2. AFM diagram for Crater Lake volcanics, Oregon Cascades. Data compiled by Rick Conrey (personal communication).

18 The major element chemistry of MORBs Originally considered to be extremely uniform, interpreted as a simple petrogenesis More extensive sampling has shown that they display a (restricted) range of compositions

19 Table Average Analyses and CIPW Norms of MORBs (BVTP Table ) The major element chemistry of MORBs Oxide (wt%) All MAR EPR IOR SiO TiO Al 2 O FeO* MgO CaO Na 2 O K 2 O P 2 O Total Norm q or ab an di hy ol mt il ap All: Ave of glasses from Atlantic, Pacific and Indian Ocean ridges. MAR: Ave. of MAR glasses. EPR: Ave. of EPR glasses. IOR: Ave. of Indian Ocean ridge glasses.

20 The major element chemistry of MORBs Figure Fenner-type variation diagrams for basaltic glasses from the Afar region of the MAR. Note different ordinate scales. From Stakes et al. (1984) J. Geophys. Res., 89,

21 Conclusions about MORBs, and the processes beneath mid-ocean ridges MORBs are not such completely uniform magmas Chemical trends consistent with fractional crystallization of olivine, plagioclase, and perhaps clinopyroxene MORBs cannot be primary magmas, but are derivative magmas resulting from fractional crystallization (up to ~ 60%)

22 Fast ridge segments (EPR) a broader range of compositions and a larger proportion of evolved liquids (magmas erupted slightly off the axis of ridges are more evolved than those at the axis itself) Figure Histograms of over 1600 glass compositions from slow and fast midocean ridges. After Sinton and Detrick (1992) J. Geophys. Res., 97,

23 For constant Mg# considerable variation is still apparent. Figure Data from Schilling et al. (1983) Amer. J. Sci., 283,

24 Incompatible-rich and incompatible-poor mantle source regions for MORB magmas N-MORB (normal MORB) taps the depleted upper mantle source Mg# > 65: K 2 O < 0.10 TiO 2 < 1.0 E-MORB (enriched MORB, also called P-MORB for plume) taps the (deeper) fertile mantle Mg# > 65: K 2 O > 0.10 TiO 2 > 1.0

25 Trace Element and Isotope Chemistry REE diagram for MORBs Figure Data from Schilling et al. (1983) Amer. J. Sci., 283,

26 E-MORBs are enriched over N-MORBs: regardless of Mg# Lack of a distinct break suggests three MORB types E-MORBs La/Sm > 1.8 N-MORBs La/Sm < 0.7 T-MORBs (transitional) intermediate values Figure Data from Schilling et al. (1983) Amer. J. Sci., 283,

27 N-MORBs: 87 Sr/ 86 Sr < and 143 Nd/ 144 Nd > , depleted mantle source E-MORBs extend to more enriched values stronger support distinct mantle reservoirs for N- type and E-type MORBs Figure Data from Ito et al. (1987) Chemical Geology, 62, ; and LeRoex et al. (1983) J. Petrol., 24,

28 Conclusions: MORBs have > 1 source region The mantle beneath the ocean basins is not homogeneous N-MORBs tap an upper, depleted mantle E-MORBs tap a deeper enriched source T-MORBs = mixing of N- and E- magmas during ascent and/or in shallow chambers

29 Experimental data: parent was multiply saturated with olivine, cpx, and opx P range = GPa (25-35 km) Figure Data from Schilling et al. (1983) Amer. J. Sci., 283,

30 Implications of shallow P range from major element data: MORB magmas = partial melting of mantle lherzolite in a rising solid diapir Melting must take place over a range of pressures P of multiple saturation = point at which melt was last in equilibrium with solid mantle Trace element and isotopic characteristics of melt reflect equilibrium distribution between melt and source reservoir (deeper for E-MORB) The major element (and hence mineralogical) character controlled by equilibrium between melt and residual mantle during rise until melt separates as a system with its own distinct character (shallow)

31 Generation Separation of plates MORB Petrogenesis Upward motion of mantle material into extended zone Decompression partial melting associated with near-adiabatic rise N-MORB melting initiated ~ km depth in upper depleted mantle where it inherits depleted trace element and isotopic char. Figure After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, and Wilson (1989) Igneous Petrogenesis, Kluwer.

32 Generation Region of melting Melt blobs separate at about km Figure After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, and Wilson (1989) Igneous Petrogenesis, Kluwer.

33 Lower enriched mantle reservoir may also be tapped by an E-MORB plume initiated near the core-mantle boundary Some ridge segments may be drawn to vigorous plumes (e.g. Iceland) Figure After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, and Wilson (1989) Igneous Petrogenesis, Kluwer.

34 Langmuir corner flow model for rising and diverging mantle passing through a triangular melting region Hotter plume (deeper origin at a) creates larger melt triangle than cooler mantle (shallower origin at b) Mantle rising nearer axis of plume traverses greater portion of triangle and thus melts more extensively Figure After Langmuir et al. (1992). AGU.

35 Table 13.3 General Differences Between Fast (> ~5 cm/a) and Slow-Spreading Ridges Fast-Spreading Ridge Ophiolite example: Semial (Oman) Axial magma chambers are more steady-state, volcanism more frequent Smoother flanks (less faulted) Symmetric and less tectonically disrupted Ridge typically higher (shallower) Longer tectonic and magmatic segments Narrow axial rise with small axial trough Wider low seismic velocity (partial melt) zone Narrow axial neovolcanic zone Thinner lithosphere (higher heat flow) Thicker, more uniform crust Extensive sheet lava flows Slightly more evolved magmas (avg. Mg# = 52.8). Less compositional diversity within areas Mantle upwelling more two-dimensional Commonly exhibit global magmatic trends of Klein and Langmuir (1987, 1989). Slow-Spreading Ridge Ophiolite example: Troodos (Cyprus) Axial magma chambers are more ephemeral and scattered, volcanism less frequent Rougher flanks (highly faulted) Commonly asymmetric, more listric & detachment faulting. Layering is less uniform. Ridge typically lower (deeper) Shorter tectonic and magmatic segments Deep discontinuous axial valleys, uplifted flanks Narrower low velocity zone- melt lens rare Wider irregular axial neovolcanic zone with more distributed local sources hills, seamounts Thicker lithosphere (lower heat flow) Thinner, less uniform crust Pillow lavas dominate extrusives Slightly less evolved magmas (avg. Mg# = 57.1). More compositional diversity within areas Mantle upwelling more three-dimensional Commonly exhibit local magmatic trends of Klein and Langmuir (1987, 1989).