Most recent models of continental growth are based on large global compilations of detrital zircon ages, which preserve a distinctly episodic record of crust formation over billion-year timescales. However, it remains unclear whether this uneven distribution of zircon ages reflects a true episodicity in the generation of continental crust through time or is an artifact of the selective preservation of crust isolated in the interior of collisional orogens. We address this issue by analyzing a new global compilation of monazite ages (n >100,000), which is comparable in size, temporal resolution, and spatial distribution to the zircon continental growth record and unambiguously records collisional orogenesis. We demonstrate that the global monazite and zircon age distributions are strongly correlated throughout most of Earth history, implying a link between collisional orogenesis and the preserved record of continental growth. Our findings support the interpretation that the continental crust provides a preservational, rather than generational, archive of crustal growth.
Resolving how and when the continental crust formed are fundamental questions of the Earth sciences that are central to understanding how the thermal and chemical differentiation of the lithosphere has influenced the evolution of our planet's atmosphere, hydrosphere, and biosphere. The mineral zircon is a critical tool for studying the growth history of continental crust, as it is ubiquitous in the intermediate to felsic rocks that dominate the continental crust, preserves a precise temporal record of crust formation as a U-Pb geochronometer, and concentrates as robust detrital grains in clastic sediments that sample large continental areas (Dhuime et al., 2017). Therefore, global compilations containing hundreds of thousands of individual detrital zircon U-Pb ages, coupled with isotopic and elemental tracers, are regarded as the most comprehensive and representative records of continental growth (Valley et al., 2005; Voice et al., 2011; Roberts and Spencer, 2015).
The most striking feature of the global detrital zircon record is the distinctly uneven temporal distribution of ages, which cluster into a series of prominent peaks and troughs (Condie et al., 2011; Cawood et al., 2013). This uneven distribution of detrital zircon record ages is interpreted by some as a primary feature that reflects changes in the volume of continental crust generated at magmatic arcs through time, suggesting that continental growth is inherently episodic (e.g., Albarède, 1998; Voice et al., 2011; Arndt, 2013; Domeier et al., 2018; Condie and Puetz, 2019). In contrast, Hawkesworth et al. (2009) highlighted that the distribution of zircon ages is also dependent on the extent to which crust is preserved in the long-term geological record and suggested that the uneven distribution of global detrital zircon ages reflects the interplay between the volume of zircon-bearing crust generated and its preservation potential in different tectonic settings. According to their model, peaks in the global detrital zircon age distribution are secondary features that reflect the selective preservation of zircon-bearing magmatic rocks insulated in the interior of continents during collisional orogenesis.
Perhaps the key observation supporting preservation bias in the continental growth record is the coincidence of global detrital zircon age peaks with periods of supercontinent assembly (Campbell and Allen, 2008; Hawkesworth et al., 2009; Cawood et al., 2013). However, this temporal link relies on an accurate understanding of the assembly histories of different supercontinents, which remain vigorously debated due to conflicting interpretations of the age, tectonic history, and correlation of ancient orogens that are now fragmented across multiple continents. We evaluate collision-induced preservational bias in the continental growth record by integrating the global monazite and detrital zircon archives. We present a new compilation of detrital, igneous, metamorphic, and hydrothermal monazite ages, which serves as a global proxy of collisional orogenesis that is independent of supercontinent reconstructions and is comparable in size, spatial distribution, and temporal resolution to the detrital zircon record of continental growth.
MONAZITE PETROGENESIS AND CONTINENTAL GROWTH
Monazite is a light rare earth element (LREE) phosphate that is a common accessory phase in metapelites and peraluminous melts. The key compositional control on monazite petrogenesis is a bulk rock chemistry enriched in Al and depleted in Ca, which otherwise favors the formation of allanite or titanite as LREE-bearing accessory phases (Spear, 2010). These compositional features are imparted to pelitic sediments through extended subaerial chemical weathering of their continental sources and are inherited by derivative melts (Fig. 1; Nesbitt and Young, 1984; Sawka et al., 1986). The formation of monazite in pelitic rocks accompanies metamorphism and partial melting at amphibolite to granulite facies (Kelsey et al., 2008; Spear, 2010)—conditions that are readily achieved during collisional thickening of sedimentary basins. Monazite is a robust Th-U-Pb geochronometer and has been routinely dated by high-throughput, in situ techniques for several decades (Parrish, 1990; Williams et al., 2007), establishing it as one of the most widely used single-mineral geochronometers.
The distinct petrogenesis of monazite and its widespread use as a geochronometer provide a high temporal resolution and global scale record of collisional orogenesis, which offers a unique opportunity to evaluate collision-induced preservational bias in the detrital zircon record of continental growth. Specifically, if the episodic distribution of global detrital zircon ages reflects true changes in the volume of continental crust produced at magmatic arcs, there is no expectation that a comparable episodicity should be observed in the monazite record because magmatic arcs typically comprise metaluminous igneous rocks and chemically immature sediments, both of which are poor sources of monazite (Fig. 1). In contrast, the selective preservation model predicts that the global monazite and zircon records should be strongly coupled and contain age peaks that coincide with episodes of collisional orogenesis, which biases the zircon record and is the primary tectonic environment in which monazite is generated.
COMPARING THE GLOBAL MONAZITE AND ZIRCON RECORDS
We compiled >100,000 individual monazite ages from metamorphic, igneous, sedimentary, and hydrothermal-related rocks from all continents and spanning the Eoarchean to Quaternary periods (Table S1 in the Supplemental Material1). Figure 2 shows histograms of monazite ages from the new compilation and the global detrital zircon ages compiled by Puetz and Condie (2019). The global distribution of monazite and detrital zircon ages is strikingly similar, with both records characterized by a large proportion of <600 Ma ages (50% and 28% of monazite and zircon ages, respectively) and prominent age populations at ca. 900–1100 Ma, 1600–2000 Ma, and 2400–2800 Ma. Unfortunately, the scarcity of published detrital monazite provenance studies precludes a robust, direct comparison of the global detrital monazite and detrital zircon age distributions (only ∼7% of ages in our compilation are from detrital grains). However, major detrital monazite and detrital zircon age populations have been shown to overlap at the scale of individual orogens with monazite ages typically being slightly younger than zircon ages (e.g., Hietpas et al., 2010; Itano et al., 2016; Mulder et al., 2019), which is consistent with the global-scale pattern evident in Figure 2.
In addition to the qualitative comparison of histograms, we performed cross-correlation analysis of the monazite and zircon age distributions following the approach outlined by Domeier et al. (2018). Cross-correlation analysis calculates the extent to which two time-series are correlated and their relative lags (i.e., the temporal offset in age peaks or troughs between two data sets). To evaluate the link between monazite and zircon age distributions and the assembly of supercontinents, we performed cross-correlation analysis of global monazite and zircon ages during the tenures of Pangea-Gondwana, Rodinia, Nuna, and a late Neoarchean supercraton (Fig. 2). The cross-correlation results can be divided into three time periods:
Prior to 3000 Ma, monazite ages lag (i.e., are younger) and are negatively correlated with zircon ages.
Between 700 Ma and 3000 Ma, monazite and zircon distributions are strongly positively correlated, with monazite ages typically lagging zircon ages by ≤60 Ma.
The 0–700 Ma interval, which includes both positive and negative cross-correlations (Fig. 2).
In detail, the third interval is characterized by strong positive correlations with monazite ages lagging zircon ages by ≤50 Ma during the Pangea-Gondwana cycle and longer lags following the breakup of Pangea (Fig. 2). The weaker negative cross-correlations for this interval are characterized by monazite ages lagging zircon ages by 50–60 Ma during the Pangea-Gondwana cycle and by 30 Ma for post-Pangean times.
To provide tectonic context for the comparison summarized in Figure 2, we present a conceptual model of continental growth describing the predicted distribution of monazite and zircon ages during a simplified Wilson Cycle driving continental convergence and divergence (Figs. 3A and 3B). This model accounts for the distinct source rocks in which monazite and zircon are generated (Fig. 1) and their preservation potential in different tectonic settings. During the subduction phase of the Wilson Cycle, subduction zone recycling balances the volume of new crust generated at magmatic arcs (Clift et al., 2009), which results in a low net-addition of zircon to the continental record. Similarly, the net-addition of monazite should be low during the subduction stage as metaluminous arc magmas contain little igneous monazite (Fig. 1), and any metamorphic monazite generated in subducted sediment has a low preservation potential. The onset of the collisional phase is marked by a peak in zircon ages, which reflects the high preservation potential of late-stage magmatic arc rocks shielded by the enveloping orogen (Hawkesworth et al., 2009). As collision proceeds, the principal episode of monazite growth accompanies metamorphism and anatexis of structurally thickened pelitic strata of the colliding passive margin. Mafic magmatism during continental breakup produces little new zircon or monazite, although the latter may be generated during alkaline felsic magmatism, metamorphism, and hydrothermal activity related to rifting (Vavra and Schaltegger, 1999).
We tested the validity of our conceptual model by comparing the age distributions of zircon and monazite from the Himalayan orogen, which is a well-characterized orogenic system that preserves the key phases of convergent margin magmatism and collisional orogenesis underpinning models advocating for preservation bias in the continental growth record (Fig. 3C). The subduction phase of the Himalayan orogen is recorded in the Gangdese batholith and related continental arc batholiths, which preserve an episodic zircon age distribution that reflects variations in magmatic flux associated with dynamic plate kinematics during the closure of the Neo-Tethys (Zhu et al., 2019). This record culminates with the largest preserved zircon age peak at ca. 50 Ma, which is approximately synchronous with the onset of continental collision (Bouilhol et al., 2013; Zhu et al., 2015). The number of zircon ages declines between 50 and 40 Ma as arc magmatism was terminated, after which a minor spread of zircon ages at 30–15 Ma records low-volume, collision-related magmatism (e.g., Searle et al., 1997). In comparison, the subduction phase of the Himalayan orogen is poorly represented in the monazite record with only ∼1% of ages falling between 120 Ma and 50 Ma (Fig. 3C). Instead, most monazite was generated during the collisional phase, which is marked by a prominent 30–10 Ma population derived from metapelites and leucogranites of the Greater Himalayan Sequence representing the collision-thickened remnants of the Indian passive margin (Harris and Massey, 1994; Parrish and Hodges, 1996).
The close agreement between the distribution of monazite and zircon ages predicted by the conceptual model (Fig. 3B) and observed in the Himalayan orogen (Fig. 3C) provides a useful framework for evaluating collision-induced preservation bias in the global record. The Himalayan case study confirms that magmatic flux at convergent margins does not appear to exert a first-order control on the distribution of monazite ages. Instead, the strong positive cross-correlation and similar time lag between monazite and zircon ages for the global data set (≤60 Ma) and the Himalayan orogen (≤40 Ma) suggests that comparable collisional orogenic events have played an important role in shaping the record of continental growth during the assembly of supercontinents since 3000 Ma. The lack of a positive correlation between monazite and zircon ages peaks prior to 3000 Ma may reflect continental growth independent of the supercontinent cycle, which is consistent with the interpretation that a global plate tectonic regime was not established at that time (Cawood et al., 2018). The decoupling of monazite and zircon age peaks prior to 3000 Ma may also reflect the scarcity of monazite-bearing lithologies on the early Earth due to the smaller volume of continental crust exposed to subaerial weathering (Fig. 1).
Our analysis also reveals a change in the relationship between the global monazite and zircon age distributions from 700 Ma onward (Fig. 2). Like earlier supercontinents, the Pangea-Gondwana cycle is characterized by strong positive correlations, with monazite ages lagging zircon ages by ≤50 Ma, reinforcing the temporal link between collisional orogenesis and global detrital zircon age peaks. The accompanying negative cross-correlations during this interval could reflect the formation of Pangea-Gondwana being less organized than that of earlier supercontinents as it may have involved the sequential assembly of a greater number of continental blocks (cf. Bradley, 2008). Such a scenario would increase the likelihood of monazite or zircon age peaks related to different orogens being offset from one another in the global compilation. Alternatively, the crustal growth record of Pangea-Gondwana may itself be biased by the fact that the Pacific Ocean, which opened during the breakup of the predecessor supercontinent Rodinia, is yet to close and has been bordered by active margins for most of its history (Cawood, 2005; Cawood and Hawkesworth, 2015). Although the pulsed record of magmatism presently preserved in the detrital zircon archive of circum-Pacific active margins is compatible with episodic continental growth models (Domeier et al., 2018), it is difficult to assess the extent to which this episodicity will be preserved upon closure of the Pacific Ocean. Similarly, the decoupling of monazite and zircon age distributions since 175 Ma may be an artifact of many post-Pangean active margins having yet to be biased by collisional orogenesis related to the assembly of a future supercontinent.
The strong correlation of global monazite and detrital zircon age distributions across most of Earth history provides compelling support for a fundamental link between collisional orogenesis and the preserved record of continental growth. Therefore, we conclude that rather than representing a purely generational archive of continental growth, the detrital zircon record has been biased by the selective preservation of crust associated with the collisional assembly of supercontinents since 3000 Ma. Although we acknowledge that the conceptual model of continental growth outlined here is necessarily simplified, the monazite archive comprises a valuable new data set that can be integrated with global compilations of other single-mineral ages, plate reconstructions, numerical models, and petrological and structural studies to achieve a more holistic understanding of the interplay between the generation and preservation of continental crust through time.
This work was funded by Australian Research Council grant FL160100168. We thank Oscar Laurent, Craig Storey, an anonymous reviewer, and editor Urs Schaltegger for insightful and constructive comments.